chore: return valid as true if no validation function provided

[JarrodMFlesch]

Description

Previously, if no validation function was passed in, many of these would return undefined when called. This causes issues inside the useField hook where it runs the validation function and isValid evaluates to undefined.

• I have read and understand the CONTRIBUTING.md document in this repository.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have added tests that prove my fix is effective or that my feature works
• Existing test suite passes locally with my changes
• I have made corresponding changes to the documentation