docs: remove null from expressions case.

Expressions don't have nulls, only doubles. If the field is missing, then its
treated as 0.0. You can query .empty to see if its missing and substitute something else.

See https://github.com/elastic/elasticsearch/pull/18132#discussion_r62068494

docs/reference/modules/scripting/expression.asciidoc

@@ -99,9 +99,9 @@ The following example shows the difference in years between the `date` fields da
 |`doc['field_name'].empty` |A boolean indicating if the field has no
 values within the doc.
 
-|`doc['field_name'].lat` |The latitude of the geo point, or `null`.
+|`doc['field_name'].lat` |The latitude of the geo point.
 
-|`doc['field_name'].lon` |The longitude of the geo point, or `null`.
+|`doc['field_name'].lon` |The longitude of the geo point.
 |=======================================================================
 
 The following example computes distance in kilometers from Washington, DC: