Emit an error if ":" is missing after a mapping key

[fktn-k]

This PR adds checks to detect errors when the ":" mapping value indicator is not followed after a mapping key.
As reported in the issue #449, the current implementation cannot detect such an invalid syntax and even generate a wrong deserialization result.
Also, the deserializer has also been fixed to properly handle empty mapping values, which was turned out to be an underlying issue of the above one.

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• Changes are described in the pull request or in a referenced issue.
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