json: better support for "type" unions (e.g. nullable arrays w/ typed items) (#7863)

* json: better suport for "type" arrays (e.g. `{"type": ["array", "null"], "items": {"type": "string"}}`)

* json: add test for type: [array, null] fix

* update tests

tests/test-grammar-integration.cpp

@@ -993,6 +993,31 @@ static void test_json_schema() {
         }
     );
 
+    test_schema(
+        "",
+        // Schema
+        R"""(
+            {
+                "type": ["array", "null"],
+                "items": { "type": "string" }
+            }
+        )""",
+        // Passing strings
+        {
+            "null",
+            "[]",
+            "[\"123\"]",
+            "[\"foo\", \"bar\"]",
+        },
+        // Failing strings
+        {
+            "",
+            "[123]",
+            "\"foo\"",
+            "[\"foo\", 42]",
+        }
+    );
+
     test_schema(
         "min+max items",
         // Schema

tests/test-json-schema-to-grammar.cpp

@@ -502,6 +502,38 @@ static void test_all(const std::string & lang, std::function<void(const TestCase
         )"""
     });
 
+    test({
+        SUCCESS,
+        "string array",
+        R"""({
+            "type": "array",
+            "prefixItems": { "type": "string" }
+        })""",
+        R"""(
+            char ::= [^"\\\x7F\x00-\x1F] | [\\] (["\\bfnrt] | "u" [0-9a-fA-F]{4})
+            root ::= "[" space (string ("," space string)*)? "]" space
+            space ::= | " " | "\n" [ \t]{0,20}
+            string ::= "\"" char* "\"" space
+        )"""
+    });
+
+    test({
+        SUCCESS,
+        "nullable string array",
+        R"""({
+            "type": ["array", "null"],
+            "prefixItems": { "type": "string" }
+        })""",
+        R"""(
+            alternative-0 ::= "[" space (string ("," space string)*)? "]" space
+            char ::= [^"\\\x7F\x00-\x1F] | [\\] (["\\bfnrt] | "u" [0-9a-fA-F]{4})
+            null ::= "null" space
+            root ::= alternative-0 | null
+            space ::= | " " | "\n" [ \t]{0,20}
+            string ::= "\"" char* "\"" space
+        )"""
+    });
+
     test({
         SUCCESS,
         "tuple1",