Add Documentation for null Locale Data

[xDivisionByZerox]

Description

Our current localization documentation does not mention the usage of null values for non applicable data.

Users should be aware of this concept as this can lead to runtime errors.

Add a section to the docs explaining the need for null values (and the resulting error).

Additional Info

• Current non-applicable data
• Introduction of the concept - refactor(locale): use null as not applicable #2078
• Related - faker.location.state() not working for sk #3052