Allow empty strings in string_or_none contract

[Joshua Blum (joshblum)]

Addresses #184

[Coveralls (coveralls)]

Coverage remained the same at 80.738% when pulling 9b57b84 on joshblum:allow-empty-strings into 91da00a on snowplow:master.

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[Joshua Blum (joshblum)]

Signed :)

[Snowplow CLA bot (snowplowcla)]

Confirmed! Joshua Blum (@joshblum) has signed the Individual Contributor License Agreement. Thanks so much

[Joshua Blum (joshblum)]

Anton Parkhomenko (@chuwy) How does this look to address #184 ??

[Joshua Blum (joshblum)]

Anton Parkhomenko (@chuwy) any update?

[Anton Parkhomenko (chuwy)]

Hey Joshua Blum (@joshblum)! Sure, this looks fine. I milestoned corresponding issue with closest release, it'll be included.

[Joshua Blum (joshblum)]

Awesome thanks :)

[Oguzhan Unlu (oguzhanunlu)]

Thanks for the contribution Joshua Blum (@joshblum) ! It is cherry-picked to the release branch at #224 .