ve.Element: Account for getComputerStyle returning null

Follows-up 143b086c74.

When the page settings dialog is opened, ve.Element.getBorders
gets called twice and in both cases, style is null in Firefox:

* HTMLLIElement
  classList: "ve-ui-widget ve-ui-optionWidget ve-ui-outlineItemWidget
   ve-ui-outlineItemWidget-level-0 ve-ui-optionWidget-selected"
  childNodes: [object HTMLSpanElement, object HTMLDivElement]

* HTMLDivElement
  classList: "ve-ui-layout ve-ui-panelLayout ve-ui-panelLayout-scrollable
   ve-ltr ve-ui-pagedDialog-outlinePanel"
  childNodes: [object HTMLUListElement]

Bug: 54322
Change-Id: I86264007c2d5b4930640a352b8896dff2d0d9ce5