Enforce that Preloaded SourceFile loaders are never created with `null` code.
Enforce that Preloaded SourceFile loaders are never created with `null` code.

PiperOrigin-RevId: 361837859

monetate/closure-compiler,ChadKillingsworth/closure-compiler,ChadKillingsworth/closure-compiler,google/closure-compiler,nawawi/closure-compiler,monetate/closure-compiler,google/closure-compiler,nawawi/closure-compiler,google/closure-compiler,google/closure-compiler,monetate/closure-compiler,nawawi/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,ChadKillingsworth/closure-compiler,monetate/closure-compiler