Normalize a phone number string to contain digits only. All non digit characters should be removed. Return empty string for empty input. Return null for null input.
Examples:
Input: phone = "(123) 456-7890"
Output: "1234567890"
Explanation: Keep digits only
Input: phone = "+1 800 555 1212"
Output: "18005551212"
Explanation: Plus signs, spaces, and hyphens are removed
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