[ASTERIXDB-3084][DOC] Fixing typo in ObjectTypeDef grammar

- user model changes: no
- storage format changes: no
- interface changes: no

details:
updating grammar in docs to allow for open-types (which we already
support)

Change-Id: I1c958781b79ac9c0ec88e0fb31f0a79370ad7ff7
Reviewed-on: https://asterix-gerrit.ics.uci.edu/c/asterixdb/+/17285
Tested-by: Jenkins <jenkins@fulliautomatix.ics.uci.edu>
Reviewed-by: Glenn Galvizo <ggalvizo@uci.edu>
diff --git a/asterixdb/asterix-doc/src/main/grammar/sqlpp.ebnf b/asterixdb/asterix-doc/src/main/grammar/sqlpp.ebnf
index b237f30..80f2bc2 100644
--- a/asterixdb/asterix-doc/src/main/grammar/sqlpp.ebnf
+++ b/asterixdb/asterix-doc/src/main/grammar/sqlpp.ebnf
@@ -162,7 +162,7 @@
 
 CreateType ::= "CREATE" "TYPE" QualifiedName ("IF" "NOT" "EXISTS")? "AS" ObjectTypeDef
 
-ObjectTypeDef ::= ("CLOSED" | "OPEN")? "{" ObjectField ("," ObjectField)* "}"
+ObjectTypeDef ::= ("CLOSED" | "OPEN")? "{" ( ObjectField ("," ObjectField)* )? "}"
 
 ObjectField ::= Identifier ":" Identifier "?"?