[Core] Replace ERR_FAIL_COND with ERR_FAIL_NULL where applicable

[AThousandShips]

• Continuation of: [Scene] Use NULL instead of COND checks when appropriate #77945

The primary reason is plain correctness, but several of these cases, especially the wider cases that will be added soon, are hard to tell what they should be with the current code, so having an explicit null check makes reading the code easier

The most straight-forward argument, taken from display_server_web.cpp:

ERR_FAIL_COND_V_MSG(!tts, false, "Enable the \"audio/general/text_to_speech\" project setting to use text-to-speech.");

And the one from display_server_windows.cpp:

ERR_FAIL_COND_V_MSG(!tts, false, "Enable the \"audio/general/text_to_speech\" project setting to use text-to-speech.");

Only one is a null check, despite being entirely identical otherwise.

Edit: Also cleaned up some error messages for style when they were already part of the change.

[AThousandShips]

Going through various areas, can make a single massive PR for all the areas or split it into dedicated parts, will start by adding the individual commits to this and will split if required

[timothyqiu]

I think the advantage of ERR_FAIL_NULL over ERR_FAIL_COND is its clear error message.

But this advantage does not exist for ERR_FAIL_NULL_MSG relative to ERR_FAIL_COND_MSG as they are basically the same except that what the parameter means is opposite.

[AThousandShips]

The advantage is communication, it makes it clear what it checks for, see the case in the OP

[aaronfranke]

Looks good from a quick review. If I understand correctly, this won't compile if it's incorrect, so I think this is pretty safe.

[AThousandShips]

Did some changes that I rolled back before which didn't show up until running (they were is_null cases) cases with but these are all pointer based, as Variant, booleans, and integers all break on compile

[AThousandShips]

The remaining parts of the engine have the following sizes in changes:

• Scene + main: 23
• Platform: 83
• Drivers: 342
• Editor: 218
• Modules: 545
• Servers: 1,104

Can combine as desired, the scan is the same so same considerations apply, servers is large, but also largely a simple pattern

All were applied with a filter in Kate, and hand picked based on the names, and rolled back a few cases that were variant or bool

[akien-mga]

Thanks!

[AThousandShips]

Thank you!