PY-17295 In the quickfix use "if param is None" check instead of "if not param"

python/src/com/jetbrains/python/inspections/quickfix/PyDefaultArgumentQuickFix.java

@@ -64,7 +64,8 @@ public class PyDefaultArgumentQuickFix implements LocalQuickFix {
       final PyNamedParameter newParam = generator.createParameter(defName, PyNames.NONE, null, languageLevel);
       param.replace(newParam);
 
-      final String conditionalText = "if not " + defName + ":\n\t" + defName + " = " + defaultValue.getText() + "\n";
+      final String conditionalText = "if " + defName + " is None:" +
+                                     "\n\t" + defName + " = " + defaultValue.getText();
       final PyIfStatement conditionalAssignment = generator.createFromText(languageLevel, PyIfStatement.class, conditionalText);
       PyUtil.addElementToStatementList(conditionalAssignment, function.getStatementList(), true);
     }

python/testData/inspections/DefaultArgumentCommentsInsideParameters_after.py

@@ -1,6 +1,6 @@
 def func(x,  # comment
          mutable=None):
     """Docstring."""
-    if not mutable:
+    if mutable is None:
         mutable = []
     print(mutable)

python/testData/inspections/DefaultArgumentEmptyList_after.py

@@ -1,3 +1,3 @@
 def bar(args=None):
-    if not args:
+    if args is None:
         args = []

python/testData/inspections/DefaultArgument_after.py

@@ -1,3 +1,3 @@
 def foo(args=None):
-    if not args:
+    if args is None:
         args = []