Fix rare error when loading mnemonic word numbers

[tadeubas]

What is this PR for?

Fix errors when Loading Mnemonic > Via Manual Input > Word Numbers and input is empty and user click on "GO" btn:

• ValueError: invalid literal for int() with base 10: ''
• ValueError: invalid literal for int() with base 16: ''
• ValueError: invalid literal for int() with base 8: ''

What is the purpose of this pull request?

• Bug fix
• New feature
• Docs update
• Other

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.91%. Comparing base (fc0e661) to head (3fd4001).
Report is 12 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #513      +/-   ##
===========================================
+ Coverage    94.86%   94.91%   +0.04%     
===========================================
  Files           75       75              
  Lines         8361     8364       +3     
===========================================
+ Hits          7932     7939       +7     
+ Misses         429      425       -4     

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[odudex]

Nice catch!