Fix the dynamic step for pipeline upload on the x-pack Heartbeat (elastic#38736)

Fix unbound BEATS_PROJECT_NAME

Signed-off-by: Alexandros Sapranidis <alexandros@elastic.co>

Author: alexsapran

.buildkite/scripts/common.sh

@@ -201,7 +201,8 @@ case "${BUILDKITE_PIPELINE_SLUG}" in
 esac
 
 check_and_set_beat_vars() {
-  if [[ -n "$BEATS_PROJECT_NAME" && "$BEATS_PROJECT_NAME" == *"x-pack/"* ]]; then
+  local BEATS_PROJECT_NAME=${BEATS_PROJECT_NAME:=""}
+  if [[ "${BEATS_PROJECT_NAME:=""}" == *"x-pack/"* ]]; then
     BEATS_XPACK_PROJECT_NAME=${BEATS_PROJECT_NAME//-/}              #remove -
     BEATS_XPACK_PROJECT_NAME=${BEATS_XPACK_PROJECT_NAME//\//_}      #replace / to _
     BEATS_XPACK_LABEL_PROJECT_NAME=${BEATS_PROJECT_NAME//\//-}      #replace / to - for labels

.buildkite/x-pack/pipeline.xpack.heartbeat.yml

@@ -1,7 +1,6 @@
 # yaml-language-server: $schema=https://raw.githubusercontent.com/buildkite/pipeline-schema/main/schema.json
 name: "beats-xpack-heartbeat"
 
-
 env:
   AWS_ARM_INSTANCE_TYPE: "t4g.xlarge"
   BEATS_PROJECT_NAME: "x-pack/heartbeat"