Omit default from usage if DefValue is empty

[AdamSLevy]

Fix #204

This change allows a user to set Flag.DefValue to an empty string to suppress the "(default %v)" printed in the usage line for custom Value types that have non-standard zero values.

For example

flags:=flag.NewFlagSet("",flag.ContinueOnError)
flags.VarPF(&myValue, "value", "", "usage").DefValue = ""

Will suppress the default message in the usage line.

[CLAassistant]

All committers have signed the CLA.

[erezpaz125]

Hi, I wanted to know if this pull request could be approved (as I see all checks have passed) It deals with an issue that came up at my workplace and it would be very helpful. Thank you!