Incorrect empty string comparisons in Client.java #999

Description
The generateTenantToken method in Client.java uses == to compare a variable to an empty string. This is not correct because == tests object references not object content.

Expected behavior
To test for an empty string .isEmpty() should be used.

Current behavior
String comparisons will always return false.

Environment (please complete the following information):

• OS: n/a
• Meilisearch version: n/a
• meilisearch-java version: 0.22.1