Fix schema inference for scalar

[regadas]

Fixes #1894

[codecov]

Codecov Report

Merging #1895 into master will decrease coverage by 1.77%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1895      +/-   ##
==========================================
- Coverage   72.61%   70.83%   -1.78%     
==========================================
  Files         200      200              
  Lines        6291     6285       -6     
  Branches      445      431      -14     
==========================================
- Hits         4568     4452     -116     
- Misses       1723     1833     +110

Impacted Files	Coverage Δ
...core/src/main/scala/com/spotify/scio/sql/Sql.scala	88.29% <100%> (-0.25%)	⬇️
.../com/spotify/scio/schemas/SchemaMaterializer.scala	98.03% <100%> (+0.03%)	⬆️
...scala/com/spotify/scio/bigquery/BigQueryUtil.scala	50% <0%> (-50%)	⬇️
...la/com/spotify/scio/bigquery/client/QueryOps.scala	0.82% <0%> (-47.11%)	⬇️
.../spotify/scio/bigquery/client/BigQueryConfig.scala	41.17% <0%> (-29.42%)	⬇️
...scala/com/spotify/scio/bigquery/client/Cache.scala	62.96% <0%> (-18.52%)	⬇️
.../spotify/scio/bigquery/BigQueryPartitionUtil.scala	40% <0%> (-17.5%)	⬇️
...la/com/spotify/scio/bigquery/client/TableOps.scala	1.76% <0%> (-16.82%)	⬇️
...com/spotify/scio/bigquery/types/BigQueryType.scala	57.89% <0%> (-15.79%)	⬇️
...la/com/spotify/scio/bigquery/client/BigQuery.scala	28.94% <0%> (-10.53%)	⬇️
... and 4 more

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