fix(core-database): ensure that delegate search deals with "undefined" as username

[j-a-m-l]

Proposed changes

Searching delegates without parameters return the delegate with username "undefined" when it should not return anything.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Test (adding missing tests or fixing existing tests)

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

[codecov-io]

Codecov Report

Merging #1831 into develop will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #1831      +/-   ##
===========================================
+ Coverage    39.68%   39.73%   +0.05%     
===========================================
  Files          354      354              
  Lines         7691     7693       +2     
  Branches      1086     1113      +27     
===========================================
+ Hits          3052     3057       +5     
+ Misses        4627     4622       -5     
- Partials        12       14       +2

Impacted Files	Coverage Δ
...ckages/core-database/src/repositories/delegates.ts	72.41% <100%> (+2.04%)	⬆️
packages/core-deployer/src/utils.ts	0% <0%> (ø)	⬆️
packages/core-logger-winston/src/formatter.ts	0% <0%> (ø)	⬆️
packages/crypto/src/utils/sort-transactions.ts	90.9% <0%> (+27.27%)	⬆️

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[j-a-m-l]

Other search endpoints return all the results when no parameter is passed, so I've updated this fix to make it consistent