sw: add stub Title impl for SwPostItField

It is advertised as a property, but it's not implemented: just avoid the
assert fail on inspecting the UNO wrapper.

Similar to commit eb3fccbd86 (sw: add stub
Title impl for SwAuthorField, 2022-05-25).

Change-Id: Idd49f8c23e2d66616346b30e1f0e04eda9da7725
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/177429
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

sw/source/core/fields/docufld.cxx

@@ -1953,6 +1953,8 @@ bool SwPostItField::QueryValue( uno::Any& rAny, sal_uInt16 nWhichId ) const
             rAny <<= OUString(OUString::number(m_nPostItId, 16).toAsciiUpperCase());
         }
         break;
+    case FIELD_PROP_TITLE:
+        break;
     default:
         assert(false);
     }