Fix check if FTL is running

[DL6ER]

What does this PR aim to accomplish?:

Fix #303

How does this PR accomplish the above?:

The currently used test is broken:

if [ -z ${ftlPID+x} ]; then

will always succeed as +x will substitute the variable by x if the variable is set.

As we have

ftlPID=$(pidof pihole-FTL)

above, the variable will never be unset. However, when FTL ist not running and the variable is empty, the substitution will kick in and the test will still succeed (although it obviously shouldn't). This test is simply wrong.

The test should read

if [ -z "${ftlPID}" ]; then

instead.

Link documentation PRs if any are needed to support this PR:

N/A

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