Do not suggest !! on expression which is always null

Related to KT-14643

idea/testData/quickfix/addExclExclCall/nullExpression.kt

@@ -0,0 +1,11 @@
+// "Add non-null asserted (!!) call" "false"
+// ACTION: Add 'toString()' call
+// ACTION: Change type of 'x' to 'String?'
+// ACTION: Remove braces from 'if' statement
+// ERROR: Type mismatch: inferred type is String? but String was expected
+
+fun foo(arg: String?) {
+    if (arg == null) {
+        val x: String = arg<caret>
+    }
+}