NextAction refactoring to eliminate sentinel arrays and pointers (#1923)

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# Description

This PR aims to refactor the NextAction declaration to achieve two
goals:

## Eliminate C-style sentinel arrays

Currently, a double pointer (`NextAction**`) approach is being used.
This an old pre-C++11 (< 2011) trick before `std::vector<>` became a
thing.
This approach is painful for developers because they constantly need to
declare their `NextAction` arrays as:
```cpp
NextAction::array(0, new NextAction("foo", 1.0f), nullptr)
```
Instead of:
```cpp
{ new NextAction("foo", 1.0f) }
```
The first argument of `NextAction::array` is actually a hack. It is used
to have a named argument so `va_args` can find the remaining arguments.
It is set to 0 everywhere but in fact does nothing. This is very
confusing to people unfamiliar with this antiquated syntax.
The last argument `nullptr` is what we call a sentinel. It's a `nullptr`
because `va_args` is looking for a `nullptr` to stop iterating. It's
also a hack and also leads to confusion.

## Eliminate unnecessary pointers for `NextAction`

Pointers can be used for several reasons, to cite a few:
- Indicate strong, absolute identity.
- Provide strong but transferable ownership (unlike references).
- When a null value is acceptable (`nullptr`).
- When copy is expensive.

`NextAction` meets none of these criteria:
- It has no identity because it is purely behavioural.
- It is never owned by anything as it is passed around and never fetched
from a registry.
- The only situations where it can be `nullptr` are errors that should
in fact throw an `std::invalid_argument` instead.
- They are extremely small objects that embark a single `std::string`
and a single `float`.

Pointers should be avoided when not strictly necessary because they can
quickly lead to undefined behaviour due to unhandled `nullptr`
situations. They also make the syntax heavier due to the necessity to
constantly check for `nullptr`. Finally, they aren't even good for
performance in that situation because shifting a pointer so many times
is likely more expensive than copying such a trivial object.

# End goal

The end goal is to declare `NextAction` arrays this way:
```cpp
{ NextAction("foo", 1.0f) }
```

> [!NOTE]
> Additional note: `NextAction` is nothing but a hacky proxy to an
`Action` constructor. This should eventually be reworked to use handles
instead of strings. This would make copying `NextAction` even cheaper
and remove the need for the extremely heavy stringly typed current
approach. Stringly typed entities are a known anti-pattern so we need to
move on from those.

src/strategy/deathknight/GenericDKNonCombatStrategy.cpp

@@ -20,17 +20,17 @@ private:
     static ActionNode* bone_shield([[maybe_unused]] PlayerbotAI* botAI)
     {
         return new ActionNode("bone shield",
-                              /*P*/ nullptr,
-                              /*A*/ nullptr,
-                              /*C*/ nullptr);
+                              /*P*/ {},
+                              /*A*/ {},
+                              /*C*/ {});
     }
 
     static ActionNode* horn_of_winter([[maybe_unused]] PlayerbotAI* botAI)
     {
         return new ActionNode("horn of winter",
-                              /*P*/ nullptr,
-                              /*A*/ nullptr,
-                              /*C*/ nullptr);
+                              /*P*/ {},
+                              /*A*/ {},
+                              /*C*/ {});
     }
 };
 
@@ -44,19 +44,18 @@ void GenericDKNonCombatStrategy::InitTriggers(std::vector<TriggerNode*>& trigger
     NonCombatStrategy::InitTriggers(triggers);
 
     triggers.push_back(
-        new TriggerNode("no pet", NextAction::array(0, new NextAction("raise dead", ACTION_NORMAL + 1), nullptr)));
+        new TriggerNode("no pet", { NextAction("raise dead", ACTION_NORMAL + 1) }));
     triggers.push_back(
-        new TriggerNode("horn of winter", NextAction::array(0, new NextAction("horn of winter", 21.0f), nullptr)));
+        new TriggerNode("horn of winter", { NextAction("horn of winter", 21.0f) }));
     triggers.push_back(
-        new TriggerNode("bone shield", NextAction::array(0, new NextAction("bone shield", 21.0f), nullptr)));
+        new TriggerNode("bone shield", { NextAction("bone shield", 21.0f) }));
     triggers.push_back(
-        new TriggerNode("has pet", NextAction::array(0, new NextAction("toggle pet spell", 60.0f), NULL)));
+        new TriggerNode("has pet", { NextAction("toggle pet spell", 60.0f) }));
     triggers.push_back(
-        new TriggerNode("new pet", NextAction::array(0, new NextAction("set pet stance", 60.0f), NULL)));
+        new TriggerNode("new pet", { NextAction("set pet stance", 60.0f) }));
 }
 
 void DKBuffDpsStrategy::InitTriggers(std::vector<TriggerNode*>& triggers)
 {
-    // triggers.push_back(new TriggerNode("improved icy talons", NextAction::array(0, new NextAction("improved icy
-    // talons", 19.0f), nullptr)));
+
 }