fix(targets): Check schema has arrived before any record

[kgpayne]

Closes #1146

[kgpayne]

cc @visch

[codecov]

Codecov Report

Merging #1770 (a4b8221) into main (5f59f38) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

❗ Current head a4b8221 differs from pull request most recent head c51b0cd. Consider uploading reports for the commit c51b0cd to get more accurate results

@@            Coverage Diff             @@
##             main    #1770      +/-   ##
==========================================
- Coverage   86.29%   86.18%   -0.11%     
==========================================
  Files          59       59              
  Lines        4960     4945      -15     
  Branches      811      809       -2     
==========================================
- Hits         4280     4262      -18     
- Misses        481      483       +2     
- Partials      199      200       +1     

Impacted Files	Coverage Δ
singer_sdk/target_base.py	92.52% <100.00%> (+1.40%)	⬆️
singer_sdk/testing/target_tests.py	94.00% <100.00%> (+0.12%)	⬆️

... and 3 files with indirect coverage changes

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[visch]

cc @visch

Looks good I'll get an issue in to migrate target-postgres once this is released