cfg-tree: Fix empty block support

When compiling a sequence, only call
cfg_tree_propagate_expr_node_properties_to_pipe() when there's a pipe
to propagate properties to: when either of first_pipe or last_pipe
non-NULL. Otherwise, fall back to the case where we create an empty
pipe by hand.

This fixes a NULL dereference at config parsing time, when the config
had empty blocks.

Reported-by: Peter Czanik <czanik@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>