use defaults in meta.json

[PSeitz]

This change allows to have unset fields in meta.json and fall back to their defaults
Currently it is required to explicitly put e.g. fieldnorms: false

[codecov-commenter]

Codecov Report

Merging #1310 (e1f4825) into main (2e255c4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head e1f4825 differs from pull request most recent head cfb5d4a. Consider uploading reports for the commit cfb5d4a to get more accurate results

@@           Coverage Diff           @@
##             main    #1310   +/-   ##
=======================================
  Coverage   94.05%   94.06%           
=======================================
  Files         230      230           
  Lines       39350    39382   +32     
=======================================
+ Hits        37010    37043   +33     
+ Misses       2340     2339    -1     

Impacted Files	Coverage Δ
src/schema/field_type.rs	83.33% <ø> (ø)
src/aggregation/mod.rs	97.96% <100.00%> (-0.02%)	⬇️
src/core/index_meta.rs	93.17% <100.00%> (+0.13%)	⬆️
src/indexer/merger.rs	98.96% <100.00%> (-0.01%)	⬇️
src/query/phrase_query/mod.rs	92.92% <100.00%> (-0.05%)	⬇️
src/schema/index_record_option.rs	100.00% <100.00%> (ø)
src/schema/text_options.rs	100.00% <100.00%> (ø)
src/fastfield/reader.rs	88.23% <0.00%> (-0.85%)	⬇️
src/store/index/mod.rs	98.37% <0.00%> (+0.54%)	⬆️
... and 1 more

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