Add is null or empty

[sirknightj]

Issue #, if available: N/A

Description of changes:

• Add a new macro to check for null string or empty string. For readability purposes,

  if (!IS_NULL_OR_EMPTY(str)) {

  is better than these two

  if (!(str == NULL || str[0] == '\0')) {
  if (str != NULL && str[0] != '\0') {

Other considerations

Why don't we just use strlen(str) > 0?

1. strlen has to traverse the entire string, so it's less efficient than a comparison with the null termination character.
2. If str is NULL, using strlen directly may result in undefined behavior.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (83334d9) 75.12% compared to head (96dedbe) 75.12%.

:exclamation: Current head 96dedbe differs from pull request most recent head 1514252. Consider uploading reports for the commit 1514252 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #228 +/- ## ======================================== Coverage 75.12% 75.12% ======================================== Files 52 52 Lines 10225 10225 ======================================== Hits 7682 7682 Misses 2543 2543 ```

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