Write a unit test demonstrating stack trace in #31135

Here is the stack trace that happens when running file.line with
mode=replace on a file that exists, but is empty, as described in
the bug report:

unit.modules.file_test.FileModuleTestCase.test_replace_line_in_empty_file  .................................................
   Traceback (most recent call last):
     File "/root/SaltStack/salt/tests/unit/modules/file_test.py", line 593, in test_replace_line_in_empty_file
       mode='replace'))
     File "/root/SaltStack/salt/salt/modules/file.py", line 1523, in line
       for line in body.split(os.linesep)])
   TypeError: expected a character buffer object

tests/unit/modules/file_test.py

@@ -571,6 +571,32 @@ class FileModuleTestCase(TestCase):
             saltenv='base')
         self.assertEqual(ret, 'This is a templated file.')
 
+    def test_replace_line_in_empty_file(self):
+        '''
+        Tests that when calling file.line with ``mode=replace``,
+        the function doesn't stack trace if the file is empty.
+        Should return ``False``.
+
+        See Issue #31135.
+        '''
+        # Create an empty temporary named file
+        empty_file = tempfile.NamedTemporaryFile(delete=False,
+                                                 mode='w+')
+
+        # Assert that the file was created and is empty
+        self.assertEqual(os.stat(empty_file.name).st_size, 0)
+
+        # Now call the function on the empty file and assert
+        # the return is False instead of stack-tracing
+        self.assertFalse(filemod.line(empty_file.name,
+                                      content='foo',
+                                      match='bar',
+                                      mode='replace'))
+
+        # Close and remove the file
+        empty_file.close()
+        os.remove(empty_file.name)
+
 
 if __name__ == '__main__':
     from integration import run_tests