Reduce noise for empty-signature in export

[FantasticoFox]

{
"signature": null, // instead of object with all empty strings
}

This helps us to keep the code clean on the guardian side. Please help us to iterate through the different aspects of the output to reduce it to minimum.

[FantasticoFox]

null signature was reverted as it was buggy.

[FantasticoFox]

Current export of the default main page after deployment:

Expected output, important if its genesis revision it should also hold a null value.

[FantasticoFox]

Verifier has been updated to support null value inblockio/aqua-verifier-js#21

[it-spiderman]

Done in #374 => @FantasticoFox

Export/import in PKC works.
However, i could not test with extenal verifier, as i dont know how to get this patch into my extension
inblockio/aqua-verifier-js#21

I tried removing and re-installing the Chrome ext, but didnt help
With this patch, verifier should presumably work, but needs to be tested before PR is merged

[FantasticoFox]

Please check with the current release 1.2.6 which supports the null value.
https://chromewebstore.google.com/detail/verifypage/gadnjidhhadchnegnpadkibmjlgihiaj?hl=en-GB

[FantasticoFox]

I tested it and it works fine with the current VerifyPage release. Thank you.

[FantasticoFox]

Re-open due to #376

[it-spiderman]

To be closed when #376 is closed