Fix show in IE11

[sandyarmstrong]

Setting element.style.display to null in IE11 has no effect.
So if the suggest widget was hidden for some reason, and then
shown again, it just stayed hidden.

This issue also prevented the "No suggestions" message from displaying.

Fixes: microsoft/monaco-editor#96

[msftclas]

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[sandyarmstrong]

I used '' for consistency with other code from this repo (see goToError.ts and extensionsList.ts), but maybe initial or unset would be more appropriate.

[alexdima]

Nice catch! Thank you!