minor #5288 Constraints - empty strings and null values (anacicconi)

This PR was merged into the 2.3 branch.

Discussion
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Constraints - empty strings and null values

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |  #4244

Commits
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2616acd Constraints - empty strings and null values

Author: weaverryan

reference/constraints/Length.rst

@@ -115,6 +115,10 @@ min
 This required option is the "min" length value. Validation will fail if
 the given value's length is **less** than this min value.
 
+It is important to notice that NULL values and empty strings are considered 
+valid no matter if the constraint required a minimum length. Validators are 
+triggered only if the value is not blank.
+
 max
 ~~~