fix(tests): update slash-command-test-simple for minLength validation

command:null is coerced to "" by ajv, which now fails minLength:1
instead of the type check.

Author: ggazzo

apps/meteor/tests/end-to-end/apps/slash-command-test-simple.ts

@@ -27,7 +27,7 @@ import { IS_EE } from '../../e2e/config/constants';
 				.expect(400)
 				.expect((res) => {
 					expect(res.body).to.have.a.property('success', false);
-					expect(res.body.error).to.include('must be string');
+					expect(res.body.error).to.include('must NOT have fewer than 1 characters');
 				})
 				.end(done);
 		});