Return empty String if URL.URL is nil

[stephen-soltesz]

This change handles String() when the URL flag value is nil. Evidently String() is called during flag parsing, which requires handling the nil case.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 774

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 81.101%

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Totals
Change from base Build 771:	0.02%
Covered Lines:	1326
Relevant Lines:	1635

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💛 - Coveralls