Add string assertions for ContainsOnlyWhitespaces and DoesNotContainOnlyWhitespaces

[mzampetakis]

No description provided.

[codecov]

Codecov Report

Merging #24 (77420b2) into master (8a747c0) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #24      +/-   ##
==========================================
+ Coverage   97.61%   97.66%   +0.04%     
==========================================
  Files          12       12              
  Lines         377      385       +8     
==========================================
+ Hits          368      376       +8     
  Misses          9        9              

Impacted Files	Coverage Δ
assert/error_messages.go	100.00% <100.00%> (ø)
assert/string.go	100.00% <100.00%> (ø)

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[sonarcloud]

SonarCloud Quality Gate failed.

0 Bugs
0 Vulnerabilities
0 Security Hotspots
2 Code Smells

No Coverage information
64.0% Duplication