Fix issue with blank keyset pagination parameters

What does this MR do?

This fixes the problem described in #227166 (closed)#.

We now make sure that a blank parameter value is being treated as if the parameter wasn't set at all. The example is &id_before=, where no value is present. This is now semantically the same as omitting the parameter alltogether.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
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• Style guides
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