Don't run dunstctl if dunst daemon isn't active

[ymeister]

Description

Running dunstctl without active dunst daemon causes a several second delay of awaiting the DBus reply with the following message:

Error org.freedesktop.DBus.Error.NoReply: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
Failed to communicate with dunst, is it running? Or maybe the version is outdated. You can try 'dunstctl debug' as a next debugging step.

The fix adds an additional check for whether there exists a running dunst process in the background before running dunstctl.

Fixes #348

Checklist:

• I have performed a self-review of my own code/checked that ShellCheck succeeds
• I have made corresponding changes to the documentation (if applicable)