Fix reporting when only the report-only CSP header is present (#38002)

This was a bit confusing at first, but the report-only only
had an effect if it was used in conjunction with the regular
CSP header. This is incorrect, as the report-only header
can be present on its own.

Additionally, there was double-logic for parsing the CSP list
values, since we can only concatenate CSP lists if we have
an initial value, which requires a concrete policy value.

Therefore, abstract that way by looping over both headers and
handling the case where initially it is `None` and, if the
CSP header is not present, still `None` when we parse
the `report-only` header.

Additionally, update a WPT test. It was expecting the image
to load, yet was showing the fail image.

Part of #4577

Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>

tests/wpt/meta/MANIFEST.json

@@ -577733,7 +577733,7 @@
       ]
      ],
      "reporting-api-report-only-sends-reports-on-violation.https.sub.html": [
-      "302025669d4417db670b6ebba18a6a49aed4e2eb",
+      "89e7357fbb9022cc9d068054151a9bf87d846276",
       [
        null,
        {}