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.

docs/content/3.manual/manual.yml

@@ -2818,8 +2818,12 @@ sections:
           not a valid or useful path expression in `.`); use `$var |
           .foo |= . + 1` instead.
 
-          If the right-hand side outputs multiple values, only the last
-          one will be used.
+          If the right-hand side outputs no values (i.e., `empty`), then
+          the left-hand side path will be deleted, as with `del(path)`.
+
+          If the right-hand side outputs multiple values, only the first
+          one will be used (NOTE: it used to be that only the last one was
+          used).
 
         examples: