parse(keep_zero: true) treats empty strings like 0

[regentgal]

[50] pry(main)> ChronicDuration.parse("   ")
=> nil
[51] pry(main)> ChronicDuration.parse("   ", keep_zero: true)
=> 0

I believe that empty strings should still be treated as invalid and return nil. There is a difference between not entering anything and explicitly entering 0.

[shepmaster]

It also treats strings like "cow" as 0