Fix macOS emrun when not passing --browser at all. (#26952)

Author: juj

emrun.py

@@ -1609,7 +1609,7 @@ def run(args):  # noqa: C901, PLR0912, PLR0915
 
   options = emrun_options = parse_args(args)
 
-  if MACOS and options.browser.endswith('.app') and not options.browser.startswith('open'):
+  if MACOS and options.browser and options.browser.endswith('.app') and not options.browser.startswith('open'):
     options.browser_args = f'--new --fresh --background -a {options.browser} {options.browser_args}'
     options.browser = 'open'