Validate if circle-ci var is unset

circle.yml

@@ -398,7 +398,7 @@ jobs:
       - run:
           name: Build Test APK
           command: |
-            if [[ $MAPBOX_DEVELOPER_CONFIG_XML ]]; then
+            if [ -n "${MAPBOX_DEVELOPER_CONFIG_XML}" ]; then
               echo "${MAPBOX_DEVELOPER_CONFIG_XML}" > platform/android/MapboxGLAndroidSDKTestApp/src/main/res/values/developer-config.xml
               make android-ui-test-arm-v7
             fi
@@ -411,7 +411,7 @@ jobs:
           name: Log in to Google Cloud Platform
           shell: /bin/bash -euo pipefail
           command: |
-            if [[ $GCLOUD_SERVICE_ACCOUNT_JSON ]]; then
+            if [ -n "${GCLOUD_SERVICE_ACCOUNT_JSON}" ]; then
               echo "${GCLOUD_SERVICE_ACCOUNT_JSON}" > secret.json
               gcloud auth activate-service-account --key-file secret.json --project android-gl-native
               rm secret.json
@@ -421,7 +421,7 @@ jobs:
           no_output_timeout: 1200
           shell: /bin/bash -euo pipefail
           command: |
-            if [[ $GCLOUD_SERVICE_ACCOUNT_JSON ]]; then
+            if [ -n "${GCLOUD_SERVICE_ACCOUNT_JSON}" ]; then
               gcloud firebase test android models list
               gcloud firebase test android run --type instrumentation \
                 --app platform/android/MapboxGLAndroidSDKTestApp/build/outputs/apk/debug/MapboxGLAndroidSDKTestApp-debug.apk \
@@ -469,7 +469,7 @@ jobs:
           name: Generate Maven credentials
           shell: /bin/bash -euo pipefail
           command: |
-            if [[ $PUBLISH_NEXUS_USERNAME ]]; then
+            if [ -n "${PUBLISH_NEXUS_USERNAME}" ]; then
               aws s3 cp s3://mapbox/android/signing-credentials/secring.gpg platform/android/MapboxGLAndroidSDK/secring.gpg
               echo "NEXUS_USERNAME=$PUBLISH_NEXUS_USERNAME
               NEXUS_PASSWORD=$PUBLISH_NEXUS_PASSWORD