Turns off validation for unset `SourceFiles` fields if `required=False` (#13856)
Turns off validation for unset `SourceFiles` fields if `required=False` (#13856)

Resolves spurious validation for most `jvm_artifacts` when running `./pants tailor`. Closes #13851.


benjyw/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,pantsbuild/pants,pantsbuild/pants,benjyw/pants,benjyw/pants