Fix NPEs on unresolved references in MixedNamesCheck

Author: cirras

delphi-checks/src/main/java/au/com/integradev/delphi/checks/MixedNamesCheck.java

@@ -49,7 +49,7 @@ public DelphiCheckContext visit(NameReferenceNode reference, DelphiCheckContext
     NameDeclaration declaration = reference.getNameDeclaration();
     NameOccurrence occurrence = reference.getNameOccurrence();
 
-    if (occurrence != null) {
+    if (declaration != null) {
       if (declaration instanceof UnitImportNameDeclaration) {
         // Checks the occurrence against the original unit declaration instead of the import
         // declaration