Fix nillability of ooSetupLastVersion

...that had been introduced with 5187d3ae
"Resolves: tdf#69042 - Add a 'What's New' infobar":

* Make the default value nil instead of an empty string.

* A nilable prop can't reuse the legacy utl::ConfigManager framework to obtain
  string prop values, so drop it from there and directly use the direct-access
  functionality in SfxViewFrame::Notify.

Change-Id: I5fc67a3d7e0fcb44a218889ecf8c69e81661567e
Reviewed-on: https://gerrit.libreoffice.org/72485
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

include/unotools/configmgr.hxx

@@ -53,8 +53,6 @@ public:
 
     static OUString getProductVersion();
 
-    static OUString getLastProductVersion();
-
     static OUString getVendor();
 
     static void storeConfigItems();

officecfg/registry/schema/org/openoffice/Setup.xcs

@@ -265,7 +265,6 @@
           <desc>Specifies the version number of the previously used product for
           whats-new comparison.</desc>
         </info>
-        <value/>
       </prop>
       <prop oor:name="ooSetupVersionAboutBox" oor:type="xs:string" oor:nillable="false">
         <info>

sfx2/source/view/viewfrm.cxx

@@ -1233,8 +1233,8 @@ void SfxViewFrame::Notify( SfxBroadcaster& /*rBC*/, const SfxHint& rHint )
                 {
                     OUString sSetupVersion = utl::ConfigManager::getProductVersion();
                     sal_Int32 iCurrent = sSetupVersion.getToken(0,'.').toInt32() * 10 + sSetupVersion.getToken(1,'.').toInt32();
-                    OUString sLastVersion = utl::ConfigManager::getLastProductVersion();
-                    if (sLastVersion.isEmpty()) sLastVersion="0.0";
+                    OUString sLastVersion
+                        = officecfg::Setup::Product::ooSetupLastVersion::get().value_or("0.0");
                     sal_Int32 iLast = sLastVersion.getToken(0,'.').toInt32() * 10 + sLastVersion.getToken(1,'.').toInt32();
                     if ((iCurrent > iLast) && !Application::IsHeadlessModeEnabled() && !bIsUITest)
                     {

unotools/source/config/configmgr.cxx

@@ -135,12 +135,6 @@ OUString utl::ConfigManager::getProductVersion() {
         "Product/ooSetupVersion");
 }
 
-OUString utl::ConfigManager::getLastProductVersion() {
-    return getConfigurationString(
-        "/org.openoffice.Setup",
-        "Product/ooSetupLastVersion");
-}
-
 OUString utl::ConfigManager::getVendor() {
     return getConfigurationString(
         "/org.openoffice.Setup",