Fix corrupted data during placeholder search

[MarinPostma]

close #1088

The placeholder data was not cleared on when deleting all documents.

[codecov]

Codecov Report

Merging #1089 (f2fe969) into master (ef6b56d) will increase coverage by 0.40%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1089      +/-   ##
==========================================
+ Coverage   76.53%   76.94%   +0.40%     
==========================================
  Files         104      104              
  Lines       12127    12146      +19     
==========================================
+ Hits         9282     9346      +64     
+ Misses       2845     2800      -45     

Impacted Files	Coverage Δ
meilisearch-core/src/update/clear_all.rs	95.65% <100.00%> (+95.65%)	⬆️
meilisearch-http/tests/documents_delete.rs	100.00% <100.00%> (ø)
meilisearch-core/src/bucket_sort.rs	81.34% <0.00%> (-0.06%)	⬇️
meilisearch-core/src/number.rs	31.78% <0.00%> (ø)
meilisearch-http/tests/common.rs	91.13% <0.00%> (+0.28%)	⬆️
meilisearch-core/src/store/mod.rs	66.24% <0.00%> (+1.27%)	⬆️
meilisearch-core/src/update/mod.rs	77.11% <0.00%> (+4.97%)	⬆️
meilisearch-http/src/routes/document.rs	99.35% <0.00%> (+6.45%)	⬆️

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[Kerollmops]

bors r+

[bors]

Build succeeded:

• Test on macos-latest
• Test on ubuntu-latest