mirror: add helper to register nullable node results (#889)

Summary:
As <https://github.com/sourcecred/sourcecred/pull/883/files#r219648511>.
It is somewhat unfortunate that this mixes a command with a query, but
the concession is acceptable in this instance, I think.

Test Plan:
Existing unit tests suffice, retaining full coverage.

wchargin-branch: mirror-register-node-field-results

src/graphql/mirror.js

@@ -362,6 +362,31 @@ export class Mirror {
     }
   }
 
+  /**
+   * Register an object corresponding to the provided `NodeFieldResult`,
+   * if any, returning the object's ID. If the provided value is `null`,
+   * no action is taken, no error is thrown, and `null` is returned.
+   *
+   * As with `registerObject`, an error is thrown if an object by the
+   * given ID already exists with a different typename.
+   *
+   * This method does not begin or end any transactions. Other methods
+   * may call this method as a subroutine in a larger transaction.
+   *
+   * See: `registerObject`.
+   */
+  _nontransactionallyRegisterNodeFieldResult(
+    result: NodeFieldResult
+  ): Schema.ObjectId | null {
+    if (result == null) {
+      return null;
+    } else {
+      const object = {typename: result.__typename, id: result.id};
+      this._nontransactionallyRegisterObject(object);
+      return object.id;
+    }
+  }
+
   /**
    * Find objects and connections that are not known to be up-to-date.
    *
@@ -674,12 +699,7 @@ export class Mirror {
       `
     );
     for (const node of queryResult.nodes) {
-      let childId = null;
-      if (node != null) {
-        const childObject = {typename: node.__typename, id: node.id};
-        this._nontransactionallyRegisterObject(childObject);
-        childId = childObject.id;
-      }
+      const childId = this._nontransactionallyRegisterNodeFieldResult(node);
       const idx = nextIndex++;
       addEntry.run({connectionId, idx, childId});
     }