improve sanitation of empty credential values to match API v1 behavior

This is mostly backwards compatability to avoid surprises: in 3.1.x
if you submit a field value with `null` or an empty string to
a CharField, it's treated as an empty string (and SSH key validation
is skipped).  For boolean field values (`net.authorize`), `null` and
empty string are coerced to `False`.

see: #7216
see: #7218

awx/main/models/credential.py

@@ -457,6 +457,13 @@ class CredentialType(CommonModelNameNotUnique):
             if field.get('ask_at_runtime', False) is True
         ]
 
+    def default_for_field(self, field_id):
+        for field in self.inputs.get('fields', []):
+            if field['id'] == field_id:
+                if 'choices' in field:
+                    return field['choices'][0]
+                return {'string': '', 'boolean': False}[field['type']]
+
     @classmethod
     def default(cls, f):
         func = functools.partial(f, cls)