Fix client's tokens lifetime and authentication method management

[enricovianello]

The current behavior is:

• setting zero or null for access token duration will be overwritten with a default value
• setting null for refresh token duration will be overwritten with a default value
• setting zero for refresh token duration will be considered as infinite
• client's UI token page will prompt default duration values for tokens
• if authentication method is set to null, on creation and on update, client secret always becomes null
• if client secret is null and authentication method requires it, it's initialized with a generated value

[sonarcloud[bot]]

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