Simplify error checking in get-otp.

Using an authenticated encryption scheme means we can check if we had
the proper password and not print anything to standard out, instead of
having jq verify if its input is valid JSON.

get-otp.in

@@ -1,14 +1,12 @@
 #!/bin/sh
 file="${OTP_ACCOUNTS:-$HOME/.local/share/otp_accounts}"
 json="$(@PREFIX@/libexec/cbc-file unlock "$file")"
+[ -z "$json" ] && exit 1
 
-pre_label="$(printf %s "$json" |
-	jq '.[].label' 2>/dev/null)"
-[ -z "$pre_label" ] && exit 1
-
-label="$(printf %s "$pre_label" |
+label="$(printf %s "$json" |
+	jq '.[].label' 2>/dev/null |
 	fzf --cycle -1 ${1:+--query "$1"}
-	)"
+)"
 [ -z "$label" ] && exit 1
 
 secret="$(printf %s "$json" |