prevent highlighting a line that contains only spaces and line breaks

[Artawower]

Hi. It's a bit annoying when selection-highlight-mode tries to highlight empty lines. This pull request is to add an optional boolean variable to control this behavior. The default value is nil for backward compatibility

Without selection-highlight-mode-disable-for-empty-line-p

With (setq selection-highlight-mode-disable-for-empty-line-p t)