screenShield: Fix use of null this._dialog

screenShield: Fix use of null this._dialog

On Fedora 32 if you close the laptop lid during gnome-initial-setup,
gnome-shell hits this error:

JS ERROR: Exception in callback for signal: prepare-for-sleep: TypeError: this._dialog is undefined
_resetLockScreen@resource:///org/gnome/shell/ui/screenShield.js:434:9
activate@resource:///org/gnome/shell/ui/screenShield.js:571:14
lock@resource:///org/gnome/shell/ui/screenShield.js:617:14
_prepareForSleep@resource:///org/gnome/shell/ui/screenShield.js:219:22
_emit@resource:///org/gnome/gjs/modules/core/_signals.js:133:47
_prepareForSleep@resource:///org/gnome/shell/misc/loginManager.js:198:14
_emit@resource:///org/gnome/gjs/modules/core/_signals.js:133:47
_convertToNativeSignal@resource:///org/gnome/gjs/modules/core/overrides/Gio.js:169:19

This is because _ensureUnlockDialog() hit its first early return. So
return early from activate() in that case, so this._dialog doesn't get
used while it's null.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1213