[bitcoin/bitcoin] [Logging] Only log «Using config file PATH_TO_bitcoin.conf» message on startup if conf file exists (#14057)

Currently we log a message indicating that a bitcoin.conf file is being used even if one does not exist. This PR changes the logic to:

If config file does not exist, log:
`Not using config file. No file exists at FILE_PATH (default or passed with flag)`.

If config file exists, log same message as before.

Note: This is a (modified) subset of changes introduced in https://github.com/bitcoin/bitcoin/pull/13761 which creates a default example config file. I think it makes sense to extract this small bit out into a separate PR.

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— Commit Summary —

* Only log «Using PATH_TO_bitcoin.conf» message on startup if conf file exists.

— File Changes —

M src/init.cpp (10)

— Patch Links —