ci: default FOSSA branch input to master

Set the workflow_dispatch branch input default to "master" so manual
dispatches without an explicit input attach the scan to the master
branch in the FOSSA dashboard. The previous empty-string default with
a github.ref_name fallback worked but was confusing in the Run-workflow
UI (the input rendered empty). Direct use of inputs.branch is now safe
because the input always carries a value.

JIRA: TRIVIAL
risk: nonprod

Author: hkad98

.github/workflows/fossa.yaml

@@ -5,9 +5,9 @@ on:
   workflow_dispatch:
     inputs:
       branch:
-        description: Branch label to attach to the FOSSA scan (defaults to the dispatched ref).
+        description: Branch label to attach to the FOSSA scan.
         required: false
-        default: ""
+        default: master
 
 concurrency:
   group: fossa-${{ github.workflow }}-${{ github.ref }}
@@ -39,7 +39,7 @@ jobs:
         uses: fossas/fossa-action@ff70fe9fe17cbd2040648f1c45e8ec4e4884dcf3  # v1.9.0
         with:
           api-key: ${{ secrets.FOSSA_API_KEY }}
-          branch: ${{ inputs.branch != '' && inputs.branch || github.ref_name }}
+          branch: ${{ inputs.branch }}
 
       - name: Run FOSSA test (policy gate)
         uses: fossas/fossa-action@ff70fe9fe17cbd2040648f1c45e8ec4e4884dcf3  # v1.9.0