Accept 0 (zero) as a value set for a number field's value option.

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• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Added additional logic to check if a value was set and is numeric to allow a default value of 0

Resolves #226

How to test the changes in this Pull Request:

1. Create a code snippet for the site that creates a number field and set the value option for the field to 0

$fields[] = new PMProRH_Field(
		'test_value_zero', // input field name, used as meta key
		'number',         // field type
		array(
			'label'           => 'Number Test', // display custom label, if not used field name will be used
			'value'           => '0',
			'hint'            => 'Testing 0 (zero) as the default value', // display a hint under field
			'profile'         => true, // show on profile
		)
	);

2. Navigate to the Membership Checkout page on the frontend.
3. Inspect rendered HTML for the number field to confirm the value is set as 0

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

BUGFIX/ENHANCEMENT: Accept 0 (zero) as the default value displayed for a number field.