Make |= delete LHS when RHS is empty (Fix #1314)

Now that #1313 is fixed, |= no longer outputs null when the RHS update
expression outputs empty.

When a user wants to keep the current value of the LHS they would have
the RHS update expression output `.`, so having `empty` achieve the same
thing would be redundant.  The obvious thing to do is to delete the LHS
when the RHS update outputs `empty` (i.e., doesn't output any values).

It's reasonable to think that existing programs won't be broken by this
change, because reduce and |= not handling empty well is clearly a bug.
(Though it's possible that some programs were using empty to quickly
terminate reduce or |=, it's not likely.  They should use label/break
instead.)

Prior to this change |= would use the _last_ value output by the RHS
update expression.  With this change |= will use the _first_ value
instead.  This change _is_ a minor backwards-incompatible change.  It
may or may not be acceptable; we'll see.  It is a useful change in that
it makes |= faster when the update expression produces multiple values.

src/builtin.jq

@@ -10,7 +10,7 @@ def min_by(f): _min_by_impl(map([f]));
 def add: reduce .[] as $x (null; . + $x);
 def del(f): delpaths([path(f)]);
 def _assign(paths; value): value as $v | reduce path(paths) as $p (.; setpath($p; $v));
-def _modify(paths; update): reduce path(paths) as $p (.; setpath($p; getpath($p) | update));
+def _modify(paths; update): reduce path(paths) as $p (.; label $out | (setpath($p; getpath($p) | update) | ., break $out), delpaths([$p]));
 def map_values(f): .[] |= f;
 
 # recurse