[swift6] fix map nullable values (#17996) (#23629)

Swift6 generator ignores `nullable: true` on `additionalProperties`
values, producing `[String: String]` instead of `[String: String?]`.
This causes `DecodingError` at runtime when the JSON payload contains
null values inside dictionaries.

Mirrors the fix applied to the Kotlin generator in #23074.

Author: darrufat

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/Swift6ClientCodegen.java

@@ -754,11 +754,17 @@ public String getTypeDeclaration(Schema p) {
             return ModelUtils.isSet(p) ? "Set<" + getTypeDeclaration(inner) + ">" : "[" + getTypeDeclaration(inner) + "]";
         } else if (ModelUtils.isMapSchema(p)) {
             Schema inner = ModelUtils.getAdditionalProperties(p);
-            return "[String: " + getTypeDeclaration(inner) + "]";
+            return "[String: " + getItemsTypeDeclaration(inner) + "]";
         }
         return super.getTypeDeclaration(p);
     }
 
+    private String getItemsTypeDeclaration(Schema items) {
+        String itemsTypeDeclaration = getTypeDeclaration(items);
+        String nullable = items.getNullable() != null && items.getNullable() && !itemsTypeDeclaration.endsWith("?") ? "?" : "";
+        return itemsTypeDeclaration + nullable;
+    }
+
     @Override
     public String getSchemaType(Schema p) {
         String openAPIType = super.getSchemaType(p);

modules/openapi-generator/src/test/java/org/openapitools/codegen/swift6/Swift6ClientCodegenTest.java

@@ -19,6 +19,7 @@
 
 import io.swagger.v3.oas.models.OpenAPI;
 import io.swagger.v3.oas.models.Operation;
+import io.swagger.v3.oas.models.media.Schema;
 import org.openapitools.codegen.*;
 import org.openapitools.codegen.config.CodegenConfigurator;
 import org.openapitools.codegen.languages.Swift6ClientCodegen;
@@ -401,4 +402,20 @@ public void oneOfDiscriminatorFirstDecodingTest() throws IOException {
             output.deleteOnExit();
         }
     }
+
+    @Test(description = "Issue #17996")
+    public void testNullableMap() {
+        final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/swift6/issue17996-nullable-map.yaml");
+
+        Schema test1 = openAPI.getComponents().getSchemas().get("NullMapNotNullMap");
+        CodegenModel cm1 = swiftCodegen.fromModel("NullMapNotNullMap", test1);
+
+        // Assert the dataType properly generated
+        CodegenProperty nullableMap = cm1.vars.get(0);
+        CodegenProperty notNullableMap = cm1.vars.get(1);
+        CodegenProperty defaultMap = cm1.vars.get(2);
+        Assert.assertEquals(nullableMap.getDataType(), "[String: String?]");
+        Assert.assertEquals(notNullableMap.getDataType(), "[String: String]");
+        Assert.assertEquals(defaultMap.getDataType(), "[String: String]");
+    }
 }

modules/openapi-generator/src/test/resources/3_0/swift6/issue17996-nullable-map.yaml

@@ -0,0 +1,22 @@
+openapi: 3.0.0
+info:
+  title: 'Issue 17996 Nullable map'
+  version: latest
+components:
+  schemas:
+    NullMapNotNullMap:
+      properties:
+        nullableMap:
+          type: object
+          additionalProperties:
+            type: string
+            nullable: true
+        notNullableMap:
+          type: object
+          additionalProperties:
+            type: string
+            nullable: false
+        defaultMap:
+          type: object
+          additionalProperties:
+            type: string