IDEA-168309 Completion inside Class.forName starts suggesting root packages again when something unknown entered

1. getCompletionContext() should not return root packages if context reference is present but cannot be resolved
2. InstanceofExpressionPostfixTemplate should not be available inside the string literals

GitOrigin-RevId: ad08e280cec09ddb6896c9d358c41cfc68b2cb8e

java/java-tests/testData/codeInsight/completion/reflection/ClassForNameInvalidPackage.java

@@ -0,0 +1,5 @@
+class Main {
+  void foo() throws ReflectiveOperationException {
+    Class.forName("com.foooooo.<caret>");
+  }
+}

java/java-tests/testData/codeInsight/completion/reflection/ClassForNameInvalidPackage_after.java

@@ -0,0 +1,5 @@
+class Main {
+  void foo() throws ReflectiveOperationException {
+    Class.forName("com.foooooo.<caret>");
+  }
+}