Fix MATRIX_COLS and MATRIX_ROWS generation for custom matrix (#19508)

The code which generated the MATRIX_COLS and MATRIX_ROWS defines from
the JSON information was checking the presence of the `matrix_pins` key,
which may not exist if a custom matrix is used.  Check the presence of
`matrix_size` instead.

lib/python/qmk/cli/generate/config_h.py

@@ -62,7 +62,7 @@ def matrix_pins(matrix_pins, postfix=''):
 def generate_matrix_size(kb_info_json, config_h_lines):
     """Add the matrix size to the config.h.
     """
-    if 'matrix_pins' in kb_info_json:
+    if 'matrix_size' in kb_info_json:
         config_h_lines.append(generate_define('MATRIX_COLS', kb_info_json['matrix_size']['cols']))
         config_h_lines.append(generate_define('MATRIX_ROWS', kb_info_json['matrix_size']['rows']))