fix: execute runValidationWhenNil validators on untyped nil values

[AdamMagued]

Summary

When validating untyped nil values (such as data[field] == nil in ValidateMap or Var(nil, tag)), the field value's kind is reflect.Invalid.

Previously, traverseField checked if kind == reflect.Invalid and immediately appended a field validation error and returned without checking ct.runValidationWhenNil.

This caused custom validators registered with callValidationEvenIfNull: true (and cross-field validators with nilCheckable: true such as required_if, required_unless, and excluded_if) to be bypassed whenever a field had an untyped nil value, erroneously reporting validation failures.

Changes

1. Unify reflect.Invalid with nullable handling in traverseField:
   • Check !ct.runValidationWhenNil before reporting an error when kind is reflect.Invalid, allowing custom validators and cross-field validators to execute as intended.
   • For typeOmitNil, treat reflect.Invalid as nil and return early.
2. Safe reflection on invalid values:
   • Ensure typ = current.Type() is only executed when current.IsValid(), avoiding panics.
   • Ensure getValue(val) returns nil when !val.IsValid(), avoiding panics on val.CanInterface().
3. Regression Tests:
   • Added TestIssue806 in validator_test.go verifying ValidateMap, Var, custom failure reporting, and cross-field validation (required_if) on untyped nil values.

Fixes #806