CppunitTest_sc_tiledrendering: ignore non-json payload for STATE_CHANGED

The payload can be both plain text and json here, but in practice we
only care about json for now, and I never saw a plain text payload when
running this suite locally.

<https://gerrit.libreoffice.org/c/core/+/161691/4#message-d4772c1362bc5e059f8ea13b4163926d9b462535>
reports that sometimes we get an .uno:InsertAnnotation=enabled, so just
ignore that without feeding that not-well-formed JSON to the JSON
parser.

Change-Id: Ic679ad20c453254237b906d4d2040824517d56e0
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/161873
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins
diff --git a/sc/qa/unit/tiledrendering/tiledrendering.cxx b/sc/qa/unit/tiledrendering/tiledrendering.cxx
index 25d1258..aa142a0 100644
--- a/sc/qa/unit/tiledrendering/tiledrendering.cxx
+++ b/sc/qa/unit/tiledrendering/tiledrendering.cxx
@@ -589,6 +589,11 @@ public:
        case LOK_CALLBACK_STATE_CHANGED:
        {
            std::stringstream aStream(pPayload);
            if (!aStream.str().starts_with("{"))
            {
                break;
            }

            boost::property_tree::ptree aTree;
            boost::property_tree::read_json(aStream, aTree);
            std::string aCommandName = aTree.get<std::string>("commandName");