Fixed the return value of the GetOfficeExePath, this should be null s…

…o it's clear the file hasn't been found.

src/Greenshot.Plugin.Office/OfficeUtils.cs

@@ -38,6 +38,6 @@ public static string GetOfficeExePath(string exeName)
                 return $@"{installRootKey.GetValue("Path")}\{exeName}";
             }
         }
-        return string.Empty;
+        return null;
     }
 }