`--update-data` introduces new blank lines when aligning multi-line expected info comments

Bug Report

After using --update-data in https://github.com/python/mypy/pull/19904/, I'm seeing a lot of diffs like this:

 [case testErrorCodeIgnoreMultiple1]
 a = 'x'.foobar(b)  # type: ignore[name-defined, attr-defined]
 a = 'x'.foobar(b)  # type: ignore[name-defined, xyz]  # E: "str" has no attribute "foobar"  [attr-defined] \
-    # N: Error code "attr-defined" not covered by "type: ignore" comment
+                                                      # N: Error code "attr-defined" not covered by "type: ignore" comment ignoring [name-defined, xyz]
 a = 'x'.foobar(b)  # type: ignore[xyz, w, attr-defined]  # E: Name "b" is not defined  [name-defined] \
-    # N: Error code "name-defined" not covered by "type: ignore" comment
+                                                         # N: Error code "name-defined" not covered by "type: ignore" comment ignoring [xyz, w, attr-defined]
+
+

it seems to apply extra whitespace at the end of the test case if it realigns a line continuation

Semi-related to https://github.com/python/mypy/issues/17465, but only a little, I think.

Start with the --update-data entry point and the testErrorCodeIgnoreMultiple1 fixture shown in the report. Reproduce the update and trace the multiline expected-info alignment; done means realigned comments remain correct without adding blank lines at the end of the test case.