fix: getAuthenticationType in EmulatorCredentials should not throw.

`EmulatorCredentials` extends the abstract `Credentials` class. The
signature of `getAuthenticationType()` function of this class does not indicate
that this API could throw, and based on [this description](https://cloud.google.com/java/docs/reference/google-auth-library/latest/com.google.auth.oauth2.OAuth2Credentials#com_google_auth_oauth2_OAuth2Credentials_getAuthenticationType__),
I don't think it should.

Fixes #2002.

google-cloud-firestore/src/main/java/com/google/cloud/firestore/FirestoreOptions.java

@@ -313,7 +313,7 @@ public static class EmulatorCredentials extends Credentials {
 
     @Override
     public String getAuthenticationType() {
-      throw new IllegalArgumentException("Not supported");
+      return "Unauthenticated";
     }
 
     @Override