fix echo FQBNS in script (espressif#8100)

Author: P-R-O-C-H-Y

.github/scripts/find_new_boards.sh

@@ -86,5 +86,5 @@ then
     echo $json_matrix
     echo "FQBNS=${json_matrix}" >> $GITHUB_ENV
 else
-    echo "FQBNS=''" >> $GITHUB_ENV
+    echo "FQBNS=" >> $GITHUB_ENV
 fi

.github/workflows/boards.yml

@@ -30,7 +30,7 @@ jobs:
   test-boards:
     needs: find-boards
     runs-on: ubuntu-latest
-    if: ${{ needs.find-boards.outputs.fqbns != '' }}
+    if: needs.find-boards.outputs.fqbns != ''
 
     env:
       REPOSITORY: |