ci(manage-visual-tests): prevent reporting if no components are impacted

The previous check was mixing strings & numbers for comparison, which
resulted in:
".github/workflows/manage-visual-tests.yml (Line: 213, Col: 13):
Conditional expression contains literal text outside replacement tokens. 
This will cause the expression to always evaluate to truthy. 
Did you mean to put the entire expression inside ${{ }}?"

This fix makes sure we compare strings and avoid relying on 
automatic coercion from GitHub.

Author: florian-sanders-cc

.github/workflows/manage-visual-tests.yml

@@ -210,7 +210,7 @@ jobs:
           LAST_ACTUAL_UPDATE: ${{ needs.run-visual-tests.outputs.last-actual-update-utc }}
 
       - name: '[Run] Generate HTML report'
-        if: ${{ steps.merge-results-and-generate-report.outputs.nb-of-impacted-components }} > 0
+        if: steps.merge-results-and-generate-report.outputs.nb-of-impacted-components > 0
         run: node ./tasks/visual-tests/generate-visual-tests-html-report.js
 
       # Expected outputs: