Allow for empty strings to be passed into FTLCONF_ environment variables

[PromoFaux]

What does this PR aim to accomplish?:

In order to set FTL's DBFILE value to empty, one has to pass in the variable as in as FTLCONF_DBFILE: "", however this was parsed wrong by the sed command which was expected one or more chars between the "s.

See https://discourse.pi-hole.net/t/installing-a-pi-hole-while-reducing-the-writing-on-the-sd-card/60519/11?u=promofaux

This fixes it by allowing the value between "s to also be empty

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[sonarcloud]

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