no number format mumbo-jumbo needed on empty format string

For every cell with CSS class attribute we tried to obtain the number format
from the number formatter and if not found tried to create it, even for empty
strings (no mso-number-format attribute present) which are never number
formats. Skip that.

sc/source/filter/html/htmlpars.cxx

@@ -2238,6 +2238,8 @@ void ScHTMLTable::DataOn( const ImportInfo& rInfo )
                     OUString aProp("mso-number-format");
                     const ScHTMLStyles& rStyles = mpParser->GetStyles();
                     const OUString& rVal = rStyles.getPropertyValue(aElem, aClass, aProp);
+                    if (!rVal.isEmpty())
+                    {
                         OUString aNumFmt = decodeNumberFormat(rVal);
 
                         nNumberFormat = GetFormatTable()->GetEntryKey(aNumFmt);
@@ -2250,6 +2252,7 @@ void ScHTMLTable::DataOn( const ImportInfo& rInfo )
                                 nNumberFormat = NUMBERFORMAT_ENTRY_NOT_FOUND;
                         }
                     }
+                }
                 break;
             }
         }