fix: dynamic pages to return null when document is not found

LIIKUNTA-441.

Return null to fix SSR rendering when the getSTaticProps returns with notFound: true.
This is null-checker fix is needed only with fallback: true -option on, 
but should not be needed at all. The fallback: 'blocking' hides the issue 
and makes it all work better.

apps/events-helsinki/src/pages/articles/[...slug].tsx

@@ -56,6 +56,10 @@ const NextCmsArticle: NextPage<{
 
   const { footerMenu } = useContext(NavigationContext);
 
+  // FIXME: Return null to fix SSR rendering for notFound-page.
+  // This is needed only with fallback: true, but should not be needed at all.
+  if (!article) return null;
+
   return (
     <MatomoWrapper>
       <RHHCPage

apps/events-helsinki/src/pages/pages/[...slug].tsx

@@ -51,6 +51,10 @@ const NextCmsPage: NextPage<{
   const { footerMenu } = useContext(NavigationContext);
   const { t: appTranslation } = useAppEventsTranslation();
 
+  // FIXME: Return null to fix SSR rendering for notFound-page.
+  // This is needed only with fallback: true, but should not be needed at all.
+  if (!page) return null;
+
   return (
     <MatomoWrapper>
       <HCRCPage

apps/hobbies-helsinki/src/pages/articles/[...slug].tsx

@@ -54,6 +54,10 @@ const NextCmsArticle: NextPage<{
   const { t: appTranslation } = useAppHobbiesTranslation();
   const { footerMenu } = useContext(NavigationContext);
 
+  // FIXME: Return null to fix SSR rendering for notFound-page.
+  // This is needed only with fallback: true, but should not be needed at all.
+  if (!article) return null;
+
   return (
     <MatomoWrapper>
       <RHHCPage

apps/hobbies-helsinki/src/pages/pages/[...slug].tsx

@@ -50,6 +50,11 @@ const NextCmsPage: NextPage<{
   } = useConfig();
   const { footerMenu } = useContext(NavigationContext);
   const { t: appTranslation } = useAppHobbiesTranslation();
+
+  // FIXME: Return null to fix SSR rendering for notFound-page.
+  // This is needed only with fallback: true, but should not be needed at all.
+  if (!page) return null;
+
   return (
     <MatomoWrapper>
       <HCRCPage

apps/sports-helsinki/src/pages/articles/[...slug].tsx

@@ -54,6 +54,10 @@ const NextCmsArticle: NextPage<{
   const { t: appTranslation } = useAppSportsTranslation();
   const { footerMenu } = useContext(NavigationContext);
 
+  // FIXME: Return null to fix SSR rendering for notFound-page.
+  // This is needed only with fallback: true, but should not be needed at all.
+  if (!article) return null;
+
   return (
     <MatomoWrapper>
       <RHHCPage

apps/sports-helsinki/src/pages/pages/[...slug].tsx

@@ -50,6 +50,11 @@ const NextCmsPage: NextPage<{
   } = useConfig();
   const { footerMenu } = useContext(NavigationContext);
   const { t: appTranslation } = useAppSportsTranslation();
+
+  // FIXME: Return null to fix SSR rendering for notFound-page.
+  // This is needed only with fallback: true, but should not be needed at all.
+  if (!page) return null;
+
   return (
     <MatomoWrapper>
       <HCRCPage