parser_json: reject empty jump/goto chain

parser_json: reject empty jump/goto chain

When parsing a verdict map json where element jumps to chain represented
as empty string.

internal:0:0-0: Error: Parsing list expression item at index 0 failed.
internal:0:0-0: Error: Invalid set elem at index 0.
internal:0:0-0: Error: Invalid set elem expression.
internal:0:0-0: Error: Parsing command array at index 2 failed.

Fixes: 586ad210368b ("libnftables: Implement JSON parser")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>