GetSpriteTexture returns a valid, non-null texture if the actor's sprite doesn't exist

[Jekyll Grim Payne]

If an actor whose sprites don't exist is spawned dynamically at runtime, its sprites seem to be remapped to TNT1A0 so it's allowed to exist but not render. Calling GetSpriteTexture() on its current sprite OR on its defaults.spawnstate.sprite returns a texture that is considered valid and non-null.

Test with the attached archive:
1. Run with doom2.wad
2. Open console and execute netevent spawntest command
3. Observe the debug prints in the console

Expected behavior: the texture must be considered null and not valid
Observed behavior: IsNull() returns false, IsValid() returns true on both textures.

ValidateSpriteFrame works as expected. However, by all accounts the texture should be null as well.