Handling empty env vars in strict mode

[navaneeth-spotnana]

We use set -o nounset and this is causing the quick install script to fail:

/tmp/use-tbls.tmp: line 21: TBLS_TMPDIR: unbound variable

Solution: https://stackoverflow.com/a/11264114/3073656

[codecov]

Codecov Report

Merging #293 (0b4d391) into master (ec213c5) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
- Coverage   66.51%   66.43%   -0.08%     
==========================================
  Files          25       25              
  Lines        4097     4097              
==========================================
- Hits         2725     2722       -3     
- Misses       1089     1091       +2     
- Partials      283      284       +1     

Impacted Files	Coverage Δ
datasource/datasource.go	40.14% <0.00%> (-2.12%)	⬇️

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[k1LoW]

@navaneeth-spotnana Nice!! Thank you for your commitment!!!