Describe how empty strings affect the validation of string constraints

[xabbuh]

For example, when an empty string is validated through the Length constraint, it is considered to be valid no matter if the constraint required a minimum length (see symfony/symfony#11943). It should be made clear that you also have to use the NotBlank constraint to disallow empty strings too.

Note that the Length constraint isn't the only constraint with this behaviour.

[xabbuh]

This does not only affect string related validators.

Quoting from #5122:

Hello,
I tried to validate an entity with an Assert\Choice constraint and discovered that it is considered valid if you pass in a NULL value. This is really counter intuitive and I did not find any information in the validation documentation about this behavior. Only after looking at the "JSR303 Bean Validation specification" which is in the first paragraph I found out about this.

There should be some kind of warning or hint that NULL values will not be checked/considered valid.

[wouterj]

I think we can add this in the table for each constraint in the reference.

[weaverryan]

This was partially handled in #5288 (just for Length)

[javiereguiluz]

If we can't come up with a list of constraints to update, we'll need to close this as "solved", ... because Length was already updated. Thanks!