Don't use zero to signal failure

[yubiuser]

What does this PR aim to accomplish?:

We use GetFTLData "dns-port" to figure out the DNS port FTL is listening on.

PADD/padd.sh

Line 423 in 1cf30ee

ftl_dns_port=$(GetFTLData "dns-port")

This port can be 0 if the DNS function of the internal dnsmasq is disabled. (https://docs.pi-hole.net/ftldns/telnet-api/#dns-port)

However, we use 0 within GetFTLData(), to signal an error when we fail to get the telnet API port (for whatever reason)

PADD/padd.sh

Lines 114 to 124 in 1cf30ee

	GetFTLData() {
	local ftl_port data
	ftl_port=$(getFTLAPIPort)
	if [ -n "$ftl_port" ]; then
	# Send command to FTL and ask to quit when finished
	data="$(echo ">$1 >quit" | nc 127.0.0.1 "${ftl_port}")"
	echo "${data}"
	else
	echo "0"
	fi
	}

We should not use a number as return code that could have another meaning. As GetFTLData "dns-port" is the only place we actually checked for 0 (read as: we never checked anywhere else) I removed the code to signal a failure on getting the API port.

Issue discovered in #307. (This PR won't fix the issue, but can help to improve investigation)

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