Fix nagging warning on regression tests

tests/highlight_regression.sh: line 17: CI: unbound variable

This shows up when running tests locally, ever since I added CI.

Relevant Stack Overflow:
https://stackoverflow.com/questions/39296472/how-to-check-if-an-environment-variable-exists-and-get-its-value

tests/highlight_regression.sh

@@ -14,7 +14,8 @@ docker run --rm \
     -v "$vol1" -v "$vol2" \
     bat-test
 
-if [ ! "$CI" ]; then
+# if $CI is unset
+if [ -z ${CI+x} ]; then
     # have git tell the effective difference between the version of the syntax
     # in HEAD/staging and the one in working dir, for all highlighted samples
     GIT_PAGER='LESS=R less' git diff -- highlighted/

tests/theme_regression.sh

@@ -15,7 +15,8 @@ docker run --rm \
     --entrypoint /tests/inner_theme_regression.sh \
     bat-test
 
-if [ ! "$CI" ]; then
+# if $CI is unset
+if [ -z ${CI+x} ]; then
     # effective difference between HEAD/staging and working dir
     GIT_PAGER='LESS=R less' git diff -- theme/