feat: PATCH semantics for MergeRequest::$externalId

- DMD-1257
- Omitting externalId from the update request now preserves the existing
  value instead of resetting it to null. Send an empty string to clear.
- DB column changed to NOT NULL DEFAULT ''.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: martinsifra

apiary.apib

@@ -10016,7 +10016,7 @@ other buckets (production buckets or buckets belongs the other development branc
 + Attributes
     + title (required, string) - title of the merge request
     + description (optional, string) - description of the merge request
-    + externalId (optional, string) - identifier of the merge request in an external system
+    + externalId (optional, string) - identifier of the merge request in an external system; omit to keep unchanged, send empty string to clear
     + autoMergeStrategy (optional, enum[string]) - auto-merge strategy
         + Members
             + `immediately`