file-operations: Fix inverted g_assert condition

The commit e2969a9c introduced an assertion statement to ensure that the g_file_get_basename returned non-NULL string. Unfortunately, the assertion has an inverted condition. This leads to crashes in the get_unique_target_file function for files that don't have the edit-name attribute set (i.e just a few GVfs backends). Let's fix the inverted condition.

Fixes: #2825 (closed)

(cherry picked from commit fcf8f9de)