Pseudo-function argument validation inconsistencies causing null crashes or misleading errors

Some pseudo-classes behave inconsistently when required arguments are missing.

Examples:

• :lang throws Cannot read properties of null (reading 'split')
• :icontains throws Cannot read properties of null (reading 'toLowerCase')
• :nth-child throws Cannot read properties of null (reading 'match')
• :has / :not without arguments can report Unknown pseudo-class instead of a clear arity error

Expected:

• Invalid selectors should fail with consistent selector-validation errors (for example, "requires an argument"), not internal null/property errors or misleading unknown-pseudo errors.

This seems to come from uneven argument validation across pseudo execution paths.

Start by reproducing the missing-argument cases for :lang, :icontains, :nth-child, :has, and :not, then trace their pseudo execution and argument-validation paths. Done means invalid selectors consistently produce clear selector-validation or arity errors instead of null/property crashes or misleading unknown-pseudo errors.