Header and RegexpHeader should not have properties that have null as default value

identified at https://github.com/checkstyle/checkstyle/pull/14180#issuecomment-1879783296
reason is same as https://github.com/checkstyle/checkstyle/issues/14216

https://checkstyle.org/checks/header/header.html#Header

>header Specify the required header specified inline. Individual header lines must be separated by the string "\n"(even on platforms with a different line separator). [String](https://checkstyle.org/property_types.html#String) null 5.0
headerFile Specify the name of the file containing the required header. [URI](https://checkstyle.org/property_types.html#URI) null 3.2

for both properties default value should be `""`.

after fix, remove special collection in SiteUtils that allows null as default values, it was introduced at https://github.com/checkstyle/checkstyle/pull/14180

Review the Header and RegexpHeader property definitions, then inspect the SiteUtils special collection mentioned in the issue. Use the linked Header documentation and referenced pull request and issue for context; done means both properties have an empty-string default and SiteUtils no longer permits null defaults.

Written by the indexing model from the issue text.