Python US Phone Number Validator Function

Write a Python function that validates US phone numbers and converts them to E.164 format.

Requirements:
- Accept a raw string input
- Return the number in E.164 format (+1XXXXXXXXXX) if valid
- Return None for missing or empty strings
- Raise ValueError with a clear error message for malformed numbers
- Handle these common formats: (555) 867-5309, 555-867-5309, 5558675309
- Process only the 10-digit core number (no extensions)

Deliverables:
- A single Python function with clear docstring
- 3 unit tests covering: valid input in different formats, empty/None input, and invalid input
- All code should be production-ready with proper error messages

Success criteria:
- Function correctly normalizes all valid formats to E.164
- Handles edge cases (empty strings, None, extra whitespace)
- Tests pass and demonstrate the function works as expected