Standardize handling of null and undefined, and add tests for that.

[vitorgamer58]

Is your feature request related to a problem? Please describe.
We must standardize the treatment of null and undefined for fields in the database.
Nowadays we don't have tests that validate undefined fields in the toEntity method, but as reported in another issue, a null field is a value, and undefined is the lack of value, and we have encountered many problems when a key is undefined, as reported: #36 and #38

Describe the solution you'd like
Add unit tests to validate the rules related to returning null or undefined, using a validation that whenever the field is null or undefined, the field itself should be returned (i.e., not doing any conversion from null to undefined or undefined to null), as reported: #37 (comment)

Describe alternatives you've considered
I considered using checker.isEmpty, since #36 is a breaking change, so it wouldn't be a problem to have multiple breaking changes in a single PR, but the problem is that empty array ([]) and empty string ("") would also be considered empty, which could generate other problems.

Additional context
I believe that this issue would be better solved if it went up in a PR together with issues #36 and #38

[vitorgamer58]

I believe the solution should be as follows:
Has null or undefined been saved to the database? then the parser must return the value itself, without doing any checking.
Was saved empty array? Returns the empty array.