[FIR] Make FirFile.annotationsContainer nullable

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

analysis/low-level-api-fir/testdata/getOrBuildFir/invalidCode/javaClassLiteral.txt

@@ -7,7 +7,6 @@ R|<local>/anyClass|
 
 FIR FILE:
 FILE: [ResolvedTo(IMPORTS)] javaClassLiteral.kt
-    [ResolvedTo(BODY_RESOLVE)] annotations container
     public final [ResolvedTo(BODY_RESOLVE)] fun main([ResolvedTo(BODY_RESOLVE)] args: R|kotlin/Array<kotlin/String>|): R|kotlin/Unit| {
         [ResolvedTo(BODY_RESOLVE)] lval anyClass: R|kotlin/Any| = R|kotlin/Any.Any|()
         R|/funOne|(<ERROR TYPE REF: Qualified expression with unexpected selector>ERROR_EXPR(Incorrect selector expression)R|<local>/anyClass|)