Create import list psi element even if no directives are present

Fix a problem where deleting last import directive via "optimize imports" could lead to incorrect psi structure resulting in CCE, see EA-64291
New imports are inserted at the head (after package directive if present) of the file (before any comments) if no imports were present
Add test for inserting import to file where a first statement is a comment
Drop code dealing with non-existing import list in ImportInsertHelperImpl
AbstractQuickFixTest: check for unexpected actions before changes to the file are made as it relies on the first line having specific format

compiler/testData/parseCodeFragment/block/packageDirective.txt

@@ -6,6 +6,8 @@ JetFile: fragment.kt
         PsiWhiteSpace(' ')
         REFERENCE_EXPRESSION
           PsiElement(IDENTIFIER)('a')
+      IMPORT_LIST
+        <empty list>
     PsiWhiteSpace('\n')
     PROPERTY
       PsiElement(val)('val')
@@ -15,4 +17,4 @@ JetFile: fragment.kt
       PsiElement(EQ)('=')
       PsiWhiteSpace(' ')
       INTEGER_CONSTANT
-        PsiElement(INTEGER_LITERAL)('1')
+        PsiElement(INTEGER_LITERAL)('1')