fix(frontend): fix source blur in search

[lionelB]

handleRouteChange reset source in state to undefined
so we need to reset it to an empty string so we can compare new value
to the one in state and bail update if there no change

fix #937

[codecov]

Codecov Report

Merging #958 into master will increase coverage by 0.03%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #958      +/-   ##
==========================================
+ Coverage   83.55%   83.59%   +0.03%     
==========================================
  Files         191      192       +1     
  Lines        1989     2011      +22     
  Branches      299      303       +4     
==========================================
+ Hits         1662     1681      +19     
- Misses        285      287       +2     
- Partials       42       43       +1

Impacted Files	Coverage Δ
...ages/code-du-travail-frontend/src/search/Search.js	55.38% <66.66%> (+1.41%)	⬆️
...ode-du-travail-data/dataset/tag_extractor/index.js	85% <0%> (ø)

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