fix: avoid throwing exception when using NetworkList without a NetworkManager (#2539)

[PitouGames]

Resolve #2539

This PR add a check that m_NetworkBehaviour is not null before calling the CanClientWrite function.

I did it exactly the same way it's already done for NetworkVariable.

With this PR, operations on the list are done and throws the anyoning warning ( NetworkVariable<T> do the same)

Changelog

• Fixed usage of NetworkList throwing exception when used without a NetworkManager in scene. (Can't do operations on NetworkList with no NetworkManager (offline) #2539)

Testing and Documentation

• No tests have been added.
• No documentation changes or additions are necessary.

[NoelStephensUnity]

@PitouGames
What this effectively does is open the possibility that an instance can write to the NetworkList even if it won't have permissions when spawned. Upon spawning, the authority's values for this NetworkList will overwrite any values set which could lead to user confusion as to why the values aren't being applied.
(same for all cases below...but I am also struggling with the existing user confusion of not being able to set the values too)

It is a good idea... just need to noodle over it a bit more. 👍

[NoelStephensUnity]

Generally speaking, this was sort of why we allow for the assignment of values upon instantiating the instance (of course, that too can get overwritten)... which the way it currently is can be problematic if you want to expose it as a property in the inspector view and/or apply values before it is spawned but after it is instantiated.
(still pondering)

[PitouGames]

Yes, it opens the possibility that an instance writes to the NetworkList even if it won't have permissions when spawned, and this is what I need.

The issue is that today, doing an operation on the list throws an uninformative NullReferenceException instead of showing the warning you added specially for this case: