The patch for Bug 1588975 specified the "open" verb to execute a target, but
the default verb is not always "open". For example, the default verb for a font
file is "preview". We should specify null verb to start the default operation.
Now we use `IShellDispatch2.ShellExecute` to ask explorer.exe to call
`ShellExecuteExW`. That method takes an optional `VARIANT` parameter as a verb.
According to https://devblogs.microsoft.com/oldnewthing/20140919-00/?p=44023,
we need to pass `VT_ERROR` to omit an optional parameter. If we pass
other values such as `nullptr` with `VT_BSTR` or `VT_EMPTY`, explorer.exe calls
`ShellExecuteExW` with the empty string `""` instead of `nullptr`, which is not
considered as a valid verb if the target file is not associated with any app.
Differential Revision: https://phabricator.services.mozilla.com/D54036
--HG--
extra : moz-landing-system : lando
