Update aws_get_environment_value to return NULL for empty values

[waahm7]

Description of changes:
aws_get_environment_value is error-prone because one of the frequent use cases is to check an environment variable, else check other source like config file. In that case, the previous implementation required us to check three cases:

• NULL -> check the alternative
• Empty -> destroy and check the alternative
• Available -> use that
  Since we don't need to differentiate between empty values and unset values, treat empty environment values as unset.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.84%. Comparing base (2add521) to head (e5e7776).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1141      +/-   ##
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+ Coverage   83.82%   83.84%   +0.01%     
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  Files          57       57              
  Lines        5991     5998       +7     
==========================================
+ Hits         5022     5029       +7     
  Misses        969      969              

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[bretambrose]

  Since we don't need to differentiate between empty values and unset values, treat empty environment values as unset.

This is a pretty big assumption since you're not only asserting it for right now, but for everything that comes in the future too. I don't really agree with this.

[graebm]

It sucks, but this is how it is with environment variables. Sometimes just the existence of the env-var is meaningful, even if it's blank. Maybe create an alternate function like aws_get_environment_variable_nonempty() that has this behavior, and scrub the codebase for places that would benefit from using that instead

[waahm7]

Yeah, I agree. We can discuss the following options in the office tomorrow.

1. Modify the existing function, and we can create a new function in the future if required, like if we have a use case where env-var can be blank.

2. Create a new function, aws_get_environment_variable_nonempty, aws_get_environment_variable_cursor, or aws_get_environment_variable_c_str. I prefer the aws_get_environment_variable_cursor if we decide to just create a new function.

3. Keep the function as it is, and validate all the uses to handle it correctly.