fix(js_interop_gen): resolve potential Null Pointer Exception and remove redundant comment

Address second round of pull request review comments:
- Add a robust early null check on symbol inside getTypeFromDeclaration to prevent runtime Null Check Operator errors when type checker symbol lookups return null.
- Utilize static variable declaration and type promotion to completely eliminate unsafe force-null assertions (!) on the resolved symbol.
- Clean up a duplicate // get a name comment on consecutive lines inside transformer.dart.