Client methods that return Boolean should return primitive and various invalid string comparisons

[fallingrock]

Description

• The Client class has various methods that return Boolean, but there is no real reason to return an object. THe methods should return primitive boolean.
• At various points strings are tested for "" using the == operator, which is not valid. equals or isEmpty should be used.

Expected behavior
No behavior anomalies, just best practices.

Current behavior
n/a

Screenshots or Logs
n/a

Environment (please complete the following information):

• OS: n/a
• Meilisearch version: n/a
• meilisearch-java version: 0.20.1