MINIFICPP-2787 Do not encrypt empty properties and parameters

[fgerlits]

This also fixes a bug: if a sensitive value was set as

  sensitive_property:

instead of

  sensitive_property: ""

in a YAML flow configuration, then the value was null instead of an empty string, and it got expanded to the string "null", and then encrypted.

https://issues.apache.org/jira/browse/MINIFICPP-2787

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