Move omit yield to end of generator

Fix `omit` generator exiting early after seeing two consecutive non-empty bracket pairs before seeing a magic comma. This does cause a regression with `trailing_comma_optional_parens2` and it still does not fix lines with multiple magic commas.

src/black/linegen.py

@@ -1231,7 +1231,6 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
                 omit.add(id(closing_bracket))
                 omit.update(inner_brackets)
                 inner_brackets.clear()
-                yield omit
 
             if (
                 prev
@@ -1246,6 +1245,7 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
             if leaf.value:
                 opening_bracket = leaf.opening_bracket
                 closing_bracket = leaf
+    yield omit
 
 
 def run_transformer(