make "role" input optional

Per Vault documentation it doesn't have to be provided,
and the auth provider's "default_role" parameter is required
precisely for this case.
https://www.vaultproject.io/api/auth/jwt

integrationTests/basic/jwt_auth.test.js

@@ -233,6 +233,15 @@ describe('jwt auth', () => {
             expect(core.exportVariable).toBeCalledWith('SECRET', 'SUPERSECRET');
         })
 
+        it('successfully authenticates as default role without specifying it', async () => {
+            when(core.getInput)
+                .calledWith('role')
+                .mockReturnValueOnce(null);
+
+            await exportSecrets();
+            expect(core.exportVariable).toBeCalledWith('SECRET', 'SUPERSECRET');
+        })
+
     });
 
 });