config-util: Change return to a single return statement in isConfigItemExists.
config-util: Change return to a single return statement in isConfigItemExists.

Also add comments to describe the isConfigItemExists function.
zulip/zulip-electron,zulip/zulip-desktop,zulip/zulip-electron,zulip/zulip-desktop,zulip/zulip-electron,zulip/zulip-desktop,zulip/zulip-desktop