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');
+        })
+
     });
 
 });

src/auth.js

@@ -25,7 +25,7 @@ async function retrieveToken(method, client) {
         case 'jwt': {
             /** @type {string} */
             let jwt;
-            const role = core.getInput('role', { required: true });
+            const role = core.getInput('role', { required: false });
             const privateKeyRaw = core.getInput('jwtPrivateKey', { required: false });
             const privateKey = Buffer.from(privateKeyRaw, 'base64').toString();
             const keyPassword = core.getInput('jwtKeyPassword', { required: false });