main/postmarketos-initramfs: hide cryptsetup not found error (MR 4204)

Hides this error when FDE is *not* enabled, and ultimately skips the
block of logic for detecting the luks partition:
        /init: line 437: cryptsetup: not found

This error has confused some users in the past, who were reporting
issues with the initramfs.

There's probably a minor speedup on non-FDE systems as well from
bailing early.

main/postmarketos-initramfs/init_functions.sh

@@ -429,6 +429,7 @@ resize_root_partition() {
 }
 
 unlock_root_partition() {
+	command -v cryptsetup >/dev/null || return
 	partition="$(find_root_partition)"
 	if cryptsetup isLuks "$partition"; then
 		# Make sure the splash doesn't interfere