cql.g: make the parser reject INSERT JSON without a JSON value

We allow inserting column values using a JSON value, eg:
```cql
INSERT INTO mytable JSON '{ "\"myKey\"": 0, "value": 0}';
```

When no JSON value is specified, the query should be rejected.

Scylla used to crash in such cases. A recent change fixed the crash
(#14706), it now fails
on unwrapping an uninitialized value, but really it should
be rejected at the parsing stage, so let's fix the grammar so that
it doesn't allow JSON queries without JSON values.

A unit test is added to prevent regressions.

Refs: #14707
Fixes: #14709

Signed-off-by: Jan Ciolek <jan.ciolek@scylladb.com>

Closes #14785

cql3/Cql.g

@@ -479,8 +479,7 @@ orderByClause[raw::select_statement::parameters::orderings_type& orderings]
     ;
 
 jsonValue returns [uexpression value]
-    :
-    | s=STRING_LITERAL { $value = untyped_constant{untyped_constant::string, $s.text}; }
+    : s=STRING_LITERAL { $value = untyped_constant{untyped_constant::string, $s.text}; }
     | m=marker         { $value = std::move(m); }
     ;
 

test/cql-pytest/test_bad_grammar.py

@@ -22,3 +22,9 @@ def test_limit_empty(cql, table1):
         cql.execute(f'SELECT * FROM {table1} LIMIT')
     with pytest.raises(SyntaxException):
         cql.execute(f'SELECT * FROM {table1} PER PARTITION LIMIT')
+
+# INSERT JSON should require a JSON value
+# Reproduces https://github.com/scylladb/scylladb/issues/14709
+def test_json_empty(cql, table1):
+    with pytest.raises(SyntaxException):
+        cql.execute(f'INSERT INTO {table1} JSON')