Validate domainpart against RFC

[kamilwaz]

According to RFC-7622, the domainpart must comply to the following statement:

After any and all normalization, conversion, and mapping of code
points as well as encoding of the string as UTF-8, a domainpart MUST
NOT be zero octets in length and MUST NOT be more than 1023 octets in
length. (Naturally, the length limits of [RFC1034] apply, and
nothing in this document is to be interpreted as overriding those
more fundamental limits.)

In a short brief, it means that the domainpart cannot be an empty string.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f040345) 83.90% compared to head (4ace714) 84.09%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
+ Coverage   83.90%   84.09%   +0.18%     
==========================================
  Files           1        1              
  Lines          87       88       +1     
==========================================
+ Hits           73       74       +1     
  Misses         14       14              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[NelsonVides]

LGTM :)