docs: additional docs for nullable arrays

docs/docs/features/request-validation.md

@@ -57,13 +57,14 @@ Huma tries to balance schema simplicity, usability, and broad compatibility with
 Fields being nullable is determined automatically but can be overridden as needed using the logic below:
 
 1. Start with no fields as nullable
-2. If a field is a pointer:
+2. If a field is a pointer (including slices):
     1. To a `boolean`, `integer`, `number`, `string`: it is nullable unless it has `omitempty`.
-    2. To an `array`, `object`: it is **not** nullable, due to complexity and bad support for `anyOf`/`oneOf` in many tools.
+    2. To an `array`: it is nullable if `huma.DefaultArrayNullable` is true.
+    3. To an `object`: it is **not** nullable, due to complexity and bad support for `anyOf`/`oneOf` in many tools.
 3. If a field has `nullable:"false"`, it is not nullable
 4. If a field has `nullable:"true"`:
-    1. To a `boolean`, `integer`, `number`, `string`: it is nullable
-    2. To an `array`, `object`: **panic** saying this is not currently supported
+    1. To a `boolean`, `integer`, `number`, `string`, `array`: it is nullable
+    2. To an `object`: **panic** saying this is not currently supported
 5. If a struct has a field `_` with `nullable: true`, the struct is nullable enabling users to opt-in for `object` without the `anyOf`/`oneOf` complication.
 
 Here are some examples:
@@ -77,7 +78,7 @@ type MyStruct1 struct {
 }
 
 // Make a specific scalar field nullable. This is *not* supported for
-// slices, maps, or structs. Structs *must* use the method above.
+// maps or structs. Structs *must* use the method above.
 type MyStruct2 struct {
     Field1 *string `json:"field1"`
     Field2 string `json:"field2" nullable:"true"`
@@ -86,6 +87,10 @@ type MyStruct2 struct {
 
 Nullable types will generate a type array like `"type": ["string", "null"]` which has broad compatibility and is easy to downgrade to OpenAPI 3.0. Also keep in mind you can always provide a [custom schema](./schema-customization.md) if the built-in features aren't exactly what you need.
 
+!!! info "Note"
+
+    Slices in Go marshal into JSON as `null` if the slice itself is `nil` rather than allocated but empty. This is why slices are nullable by default. See the [Go JSON package documentation](https://pkg.go.dev/encoding/json#Marshal) for more information.
+
 ## Validation Tags
 
 The following additional tags are supported on model fields:

schema_test.go

@@ -722,6 +722,46 @@ func TestSchema(t *testing.T) {
 				"required": ["int"]
 			}`,
 		},
+		{
+			name: "field-nullable-array",
+			input: struct {
+				Int []int64 `json:"int" nullable:"true"`
+			}{},
+			expected: `{
+				"type": "object",
+				"additionalProperties": false,
+				"properties": {
+					"int": {
+						"type": ["array", "null"],
+						"items": {
+							"type": "integer",
+							"format": "int64"
+						}
+					}
+				},
+				"required": ["int"]
+			}`,
+		},
+		{
+			name: "field-non-nullable-array",
+			input: struct {
+				Int []int64 `json:"int" nullable:"false"`
+			}{},
+			expected: `{
+				"type": "object",
+				"additionalProperties": false,
+				"properties": {
+					"int": {
+						"type": "array",
+						"items": {
+							"type": "integer",
+							"format": "int64"
+						}
+					}
+				},
+				"required": ["int"]
+			}`,
+		},
 		{
 			name: "field-nullable-struct",
 			input: struct {