[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

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/BuilderConfigurator.kt

@@ -41,6 +41,10 @@ object BuilderConfigurator : AbstractBuilderConfigurator<FirTreeBuilder>(FirTree
             fields from klass without listOf("symbol", "resolvePhase", "resolveState", "controlFlowGraphReference")
         }
 
+        builder(file) {
+            defaultNull("annotationsContainer")
+        }
+
         builder(regularClass) {
             parents += classBuilder
             parents += typeParameterRefsOwnerBuilder

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/ImplementationConfigurator.kt

@@ -609,7 +609,7 @@ object ImplementationConfigurator : AbstractFirTreeImplementationConfigurator()
 
         impl(file) {
             default("annotations") {
-                value = "annotationsContainer.annotations"
+                value = "annotationsContainer?.annotations ?: emptyList()"
                 withGetter = true
             }
         }

compiler/fir/tree/tree-generator/src/org/jetbrains/kotlin/fir/tree/generator/NodeConfigurator.kt

@@ -477,7 +477,7 @@ object NodeConfigurator : AbstractFieldConfigurator<FirTreeBuilder>(FirTreeBuild
         }
 
         file.configure {
-            +field("annotationsContainer", fileAnnotationsContainer).withTransform()
+            +field("annotationsContainer", fileAnnotationsContainer, nullable = true).withTransform()
             +field("packageDirective", packageDirective)
             +fieldList(import).withTransform()
             +declarations.withTransform()