Previously, when no file annotations were present, the FIR element
didn't have a source.
By making it nullable, it will only be created when appropriate and the
source will never be null.
#KT-55835
We can avoid body resolution in some cases because it is more
than out contract is required
So ANNOTATIONS_ARGUMENTS_MAPPING should be enough for all
non-body parts of a declaration to return fully-resolved FirElement
Body parts incudes: body and default values
^KT-59266 Fixed