[test] lit.cfg: Don't fail if there are no availability macros defined

Author: lorentey

test/lit.cfg

@@ -432,7 +432,7 @@ lit_config.note('Compiling with -swift-version ' + swift_version)
 config.swift_test_options = '-swift-version ' + swift_version
 
 # Define availability macros for known stdlib releases.
-for macro in lit_config.params.get('availability_macros').split(";"):
+for macro in lit_config.params.get('availability_macros', '').split(";"):
     config.swift_frontend_test_options += " -define-availability '{0}'".format(macro)
     config.swift_driver_test_options += " -Xfrontend -define-availability -Xfrontend '{0}'".format(macro)