fix coverage script

Author: blink1073

.evergreen/combine-coverage.sh

@@ -8,7 +8,7 @@ set -o errexit  # Exit the script with error if any of the commands fail
 
 . .evergreen/utils.sh
 
-if [ -z "$PYTHON_BINARY" ]; then
+if [ -z "${PYTHON_BINARY:-}" ]; then
     PYTHON_BINARY=$(find_python3)
 fi