Prevent sqlite3 warning output

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Hi. When using sqlite3 version 3.34.1 I get the following output in the *dash-docs-errors* buffer:

----------------
 HEY! This is dash-docs (sqlite) error logging. If you want to disable it, set `dash-docs-enable-debugging` to nil
---------------- 


cannot open: "''"

cannot open: "''"

cannot open: "''"

cannot open: "''"

This PR fixes this behavior by using null-device ("/dev/null" when using linux) instead of '' for the -init argument of sqlite3.