Postgres: Properly handle undefined field values

[dplewis]

Prevents UnhandledPromiseRejectionWarning: Unhandled promise rejection when building queries.

This specifically happens when passing installationId=undefined on _Session

[codecov]

Codecov Report

Merging #4186 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #4186      +/-   ##
=========================================
+ Coverage   92.19%   92.2%   +0.01%     
=========================================
  Files         116     116              
  Lines        8085    8085              
=========================================
+ Hits         7454    7455       +1     
+ Misses        631     630       -1

Impacted Files	Coverage Δ
...dapters/Storage/Postgres/PostgresStorageAdapter.js	96.5% <100%> (ø)	⬆️
src/RestWrite.js	93.34% <0%> (+0.18%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 406a21e...844b5f0. Read the comment docs.