This apparently always wanted to check that _rChars.trim() is non-empty

...and d3e89269 "initial checkin -
implementations for formlayer import/export - still under construction" just
forgot the '.getLength()' in

  OSL_ENSURE(0 == _rChars.trim(), ...

that is present in other, similar code.  (And the current code happend to use
the operator ==(sal_Unicode const *, OUString const &) overload that happens to
treat a null first argument like an empty string.)

Change-Id: I9d74b6ae29ca5f5f80391de50e4898add6bf6fe2

xmloff/source/forms/propertyimport.cxx

@@ -388,7 +388,7 @@ SvXMLImportContext* OPropertyElementsContext::CreateChildContext(sal_uInt16 _nPr
 
     void OPropertyElementsContext::Characters(const OUString& _rChars)
     {
-        OSL_ENSURE(nullptr == _rChars.trim(), "OPropertyElementsContext::Characters: non-whitespace characters detected!");
+        OSL_ENSURE(_rChars.trim().isEmpty(), "OPropertyElementsContext::Characters: non-whitespace characters detected!");
         SvXMLImportContext::Characters(_rChars);
     }