Bug/fixing unbound variable 63

[justin-octo]

Using -z in an if statement causes an unbound variable error if set -eu is declared but the variable has not been declared. Changing thing to -n which checks if the variable is declared and/or undefined. undefined is an empty string like bob=""
undeclared is if it has never been declared at all

Resolves #63

[justin-octo]

@gcagle3 not sure how we missed this.

[justin-octo]

Tested this with a simple script:

#!/bin/bash

set -eu

if [[ -z $bob ]]; then
  echo "It is there with -z"
else
  echo "Fail on -z"
fi

if [[ -n $bob ]]; then
  echo "It is there with -n"
else
  echo "Fail on -n"
fi

echo "Bob is ${bob}"

Which fails with these results:

./testingif.sh: line 5: bob: unbound variable

If you add in a line into the script above the if statements:

bob=""

Then you get this result (with no error message, and the script completes):

It is there with -z
Fail on -n
Bob is 

This indicates we need -n in the if statement to properly handle when the variable GRUNTWORK_BASH_COMMONS_IMDS_VERSION is undeclared.

[gcagle3]

@gcagle3 not sure how we missed this.

Excellent catch, thank you for creating this PR! I think missing this is a side effect of the limitations of the test cases. I'm going to create a task to research improving that situation to help avoid future issues being missed.

[gcagle3]

This has been merged and included in release v0.2.1. We have also updated the release notes for v0.2.0 to include a warning about the bug.