chore: Use double quotes as default empty value

Author: lqiu96

.github/workflows/dependency_compatibility_test.yaml

@@ -10,7 +10,7 @@ on:
         description: 'Comma-separated list of dependencies to test (Format: dep1=1.0,dep2=2.0).
                       No input (default) will run the the upper-bound dependencies file.'
         required: false
-        default: ' '
+        default: ""
   schedule:
     - cron: '0 1 * * *' # Nightly at 1am
 
@@ -26,7 +26,7 @@ jobs:
           java-version: '21'
           distribution: 'temurin'
           cache: maven
-      - run: echo "DEPENDENCIES_LIST=${{ github.event.inputs.dependencies-list || ' ' }}" >> "$GITHUB_ENV"
+      - run: echo 'DEPENDENCIES_LIST=${{ github.event.inputs.dependencies-list || "" }}' >> $GITHUB_ENV
       # Set up the for the CI
       - name: Install sdk-platform-java's modules
         # gapic-generator-java requires Java 8 and is irrelevant for this CI
@@ -36,7 +36,7 @@ jobs:
       - name: Perform Dependency Compatibility Testing
         shell: bash
         run: |
-          if [[ ${{ env.DEPENDENCIES_LIST }} == ' ' ]]; then
+          if [[ ${{ env.DEPENDENCIES_LIST }} == "" ]]; then
             ./.github/scripts/test_dependency_compatibility.sh -f dependencies.txt
           else
             ./.github/scripts/test_dependency_compatibility.sh -l ${{ env.DEPENDENCIES_LIST }}
@@ -58,7 +58,7 @@ jobs:
       - name: Perform Dependency Compatibility Testing (Showcase)
         shell: bash
         run: |
-          if [[ ${{ env.DEPENDENCIES_LIST }} == ' ' ]]; then
+          if [[ ${{ env.DEPENDENCIES_LIST }} == "" ]]; then
             ../.github/scripts/test_dependency_compatibility.sh -f ../dependencies.txt
           else
             ../.github/scripts/test_dependency_compatibility.sh -l ${{ env.DEPENDENCIES_LIST }}