R4R: allow generation of gentxs with empty memo field

[alessio]

Let gentx generation continue when gaiad is unable to retrieve
machine's external IP and print a warning message.

Closes: #3424

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #3478 into develop will decrease coverage by 0.08%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop    #3478      +/-   ##
===========================================
- Coverage    59.51%   59.42%   -0.09%     
===========================================
  Files          135      135              
  Lines         9981     9982       +1     
===========================================
- Hits          5940     5932       -8     
- Misses        3669     3678       +9     
  Partials       372      372