Mitigate breaking change on 11.0 version

[deandreamatias]

• The checkNullOrEmpty breaks a lot (more than expected) the use of package
• With checkNullOrEmpty: true, the default behaviour, the use of FormBuilderValidators.required() won't make sense
• Maybe could be mitigated with dart fix as described in Data-driven Fixes.

Related with: #116

@techouse @martijn00 can continue the discussion here I guess

[ArturAssisComp]

I suggested an API change in #128. It removes the necessity of checkNullOrEmpty. What do you think?

[techouse]

I like the idea.

[deandreamatias]

I think that is a nice way to improve the package

[ArturAssisComp]

Thus I will open a draft PR to start implementing a proof of concept for this idea. I will implement it under the hood of the current API to check if it is feasible, making it possible to use all the test cases already implemented. And then, if it works properly, it can be exposed to the final users. What do you think? @deandreamatias @martijn00

[deandreamatias]

I wil really appreciate if you do this