[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/innerDeclarationsResolve/lambdaInImplicitFunBody.fir.txt

@@ -1,5 +1,4 @@
 FILE: [ResolvedTo(IMPORTS)] lambdaInImplicitFunBody.kt
-    [ResolvedTo(BODY_RESOLVE)] annotations container
     public final inline [ResolvedTo(BODY_RESOLVE)] fun <[ResolvedTo(BODY_RESOLVE)] T, [ResolvedTo(BODY_RESOLVE)] R> with([ResolvedTo(BODY_RESOLVE)] receiver: R|T|, [ResolvedTo(BODY_RESOLVE)] block: R|T.() -> R|): R|R| {
         ^with R|<local>/block|.R|SubstitutionOverride<kotlin/Function1.invoke: R|R|>|(R|<local>/receiver|)
     }