[JENKINS-76231] Fix NullPointerException when Content-Type header is missing (#219)

* Fix NullPointerException when handling responses without Content-Type header

* Add test case for handling responses without Content-Type header

Author: Fluder-Paradyne

src/main/java/jenkins/plugins/http_request/ResponseContentSupplier.java

@@ -124,9 +124,10 @@ public InputStream getContentStream() {
     private void readCharset(ClassicHttpResponse response) {
         Charset charset = null;
 
+        Header contentTypeHeader = response.getFirstHeader(HttpHeaders.CONTENT_TYPE);
         ContentType contentType = ContentType.parse(response.getEntity() != null ?
                         response.getEntity().getContentType() :
-                        response.getFirstHeader(HttpHeaders.CONTENT_TYPE).getValue());
+                        (contentTypeHeader != null ? contentTypeHeader.getValue() : null));
         if (contentType != null) {
             charset = contentType.getCharset();
             if (charset == null) {

src/test/java/jenkins/plugins/http_request/HttpRequestTest.java

@@ -990,6 +990,18 @@ void responseContentSupplierHeadersCaseInsensitivity() throws Exception {
         respSupplier.close();
     }
 
+    @Test
+    void responseContentSupplierHandlesNoContentTypeHeader() throws Exception {
+        // Prepare test context - 204 No Content response without Content-Type header
+        CloseableHttpResponse response = HttpResponseAdapter.adapt(new BasicClassicHttpResponse(204, "No Content"));
+        // Don't set entity or Content-Type header to simulate GitHub API 204 response
+        // Run test
+        ResponseContentSupplier respSupplier = new ResponseContentSupplier(ResponseHandle.NONE, response);
+        // Check expectations - should not throw NullPointerException
+        assertEquals(204, respSupplier.getStatus());
+        respSupplier.close();
+    }
+
     @Test
     void testFileUpload() throws Exception {
         // Prepare the server