Fix saving UI settings without changes

[josegar74]

The UI settings form has always the Save button enabled and clicking it without changes, returns the following error:

JSON parse error: Cannot deserialize value of type `java.lang.String` from Object value (token 
`JsonToken.START_OBJECT`); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: 
Cannot deserialize value of type `java.lang.String` from Object value (token `JsonToken.START_OBJECT`) at [Source: 
REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); line: 1, column: 29] (through reference 
chain: org.fao.geonet.domain.UiSetting["configuration"])

This pull request fix the issue.

A better solution would be to disable the button if there are no changes, but the current form is not ready for this and it will require to model the UI settings in javascript.

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