Make rtl::O[U]String::isEmpty return true bool

...instead of sal_Bool, to work around MSVC warning C4805: unsafe mix of type
'sal_Bool' and type 'bool'.

Change-Id: Ia3477539ccc23aa9daec49d633f023d414f2b4bf

sal/inc/rtl/string.hxx

@@ -320,12 +320,9 @@ public:
 
       @since LibreOffice 3.4
     */
-    sal_Bool isEmpty() const SAL_THROW(())
+    bool isEmpty() const SAL_THROW(())
     {
-        if ( pData->length )
-            return sal_False;
-        else
-            return sal_True;
+        return pData->length == 0;
     }
 
     /**

sal/inc/rtl/ustring.hxx

@@ -405,12 +405,9 @@ public:
 
       @since LibreOffice 3.4
     */
-    sal_Bool isEmpty() const SAL_THROW(())
+    bool isEmpty() const SAL_THROW(())
     {
-        if ( pData->length )
-            return sal_False;
-        else
-            return sal_True;
+        return pData->length == 0;
     }
 
     /**