Do not replace the literal string '0' with ''

[fabzzap]

In Perl, the literal string '0' is false.

Because of this, if one uses the scalar ref "0", SQL::Abstract considers it a 'false' string and replaces it with an empty string. This patch avoids that, every string that at least one non-whitespache character is used.

[codecov-io]

Codecov Report

Merging #14 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #14   +/-   ##
=======================================
  Coverage   83.06%   83.06%           
=======================================
  Files           3        3           
  Lines         927      927           
  Branches      189      189           
=======================================
  Hits          770      770           
  Misses        107      107           
  Partials       50       50

Impacted Files	Coverage Δ
lib/SQL/Abstract.pm	82.28% <100%> (ø)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 23

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 88.565%

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Files with Coverage Reduction	New Missed Lines	%
lib/SQL/Abstract.pm	1	89.6%

Totals
Change from base Build 15:	0.0%
Covered Lines:	821
Relevant Lines:	927

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💛 - Coveralls