Checks for required variables/secrets don't catch empty GitHub secrets

The checks for required variables in deploy_tre_reusable.yml only check for '', however GitHub passes empty secret values as the string 'null' which results in errors deploying the TRE (it tries to use a TRE_ID of 'null', which causes an error because it tries to set a tag value of 'null', which Azure does not allow). Adding a GitHub secret named TRE_ID but not setting any value triggers the error.

Changing the checks to the following format correctly detects missing secret values:

          if [[ "${{ secrets.AAD_TENANT_ID }}" == '' ||  "${{ secrets.AAD_TENANT_ID }}" == 'null' ]]; then
            echo "Missing secret: AAD_TENANT_ID" && exit 1
          fi

Start in deploy_tre_reusable.yml and inspect the required variable and secret checks, especially the validation for TRE_ID and AAD_TENANT_ID. Confirm how empty GitHub secrets are represented, then update the checks consistently and verify that unset secrets are rejected before deployment rather than passed to Azure.