Denote every field which is omitted when None.

There are some fields that can be sent with `null` values over JSON, which
means they are set to `None` within Python. The broad config-based exclude
directive is not given any information about the field, only it's value, so
we can't use it to determine if a field should be excluded or not in the dict
that gets built.

Instead we create a helper function to build a field definition that has the
necessary exclude function. Any field that does not use this helper function
is permitted to be None/null. This is true of the MailboxGet.ids field in this
commit. A new test is added to maintain the behavior over time.

jmapc/models/mailbox.py

@@ -5,33 +5,33 @@
 
 from dataclasses_json import config
 
-from ..serializer import Model
+from ..serializer import Model, null_omitted_field
 from .models import Operator, StrOrRef
 
 
 @dataclass
 class Mailbox(Model):
-    id: Optional[str] = field(metadata=config(field_name="id"), default=None)
-    name: Optional[str] = None
+    id: Optional[str] = null_omitted_field(metadata=config(field_name="id"))
+    name: Optional[str] = null_omitted_field()
     sort_order: Optional[int] = 0
-    total_emails: Optional[int] = None
-    unread_emails: Optional[int] = None
-    total_threads: Optional[int] = None
-    unread_threads: Optional[int] = None
+    total_emails: Optional[int] = null_omitted_field()
+    unread_emails: Optional[int] = null_omitted_field()
+    total_threads: Optional[int] = null_omitted_field()
+    unread_threads: Optional[int] = null_omitted_field()
     is_subscribed: Optional[bool] = False
-    role: Optional[str] = None
-    parent_id: Optional[str] = field(
-        metadata=config(field_name="parentId"), default=None
+    role: Optional[str] = null_omitted_field()
+    parent_id: Optional[str] = null_omitted_field(
+        metadata=config(field_name="parentId"),
     )
 
 
 @dataclass
 class MailboxQueryFilterCondition(Model):
-    name: Optional[StrOrRef] = None
-    role: Optional[StrOrRef] = None
-    parent_id: Optional[StrOrRef] = None
-    has_any_role: Optional[bool] = None
-    is_subscribed: Optional[bool] = None
+    name: Optional[StrOrRef] = null_omitted_field()
+    role: Optional[StrOrRef] = null_omitted_field()
+    parent_id: Optional[StrOrRef] = null_omitted_field()
+    has_any_role: Optional[bool] = null_omitted_field()
+    is_subscribed: Optional[bool] = null_omitted_field()
 
 
 @dataclass