Hi,
found some minor inconsistency between initial texture values for materials.
Standard one:
private _bumpTexture: Nullable<BaseTexture>;
PBR:
protected _bumpTexture: Nullable<BaseTexture> = null;
both are null or BaseTexture by type, but PBR will return null in this case but Standard undefined;
example is only about bumpTexture but there are more places like this.
Not very important, found out only because my unit tests were broken 
They should all be null by default