fix: do not generate a coverage diff without coverage artifact (FuelLabs#1629)

Author: danielbate

.changeset/rotten-seas-battle.md

@@ -0,0 +1,2 @@
+---
+---

.github/workflows/test.yaml

@@ -113,6 +113,7 @@ jobs:
           path: coverage-master
 
       - name: Generate Coverage Diff
+        if: ${{ (steps.findPr.outputs.number) }}
         run: pnpm test:coverage-diff
 
       - name: Report Coverage