fix GHNotificationStream "Unable to parse If-Modified-Since request header"

src/main/java/org/kohsuke/github/GHNotificationStream.java

@@ -187,7 +187,9 @@ GHThread fetch() {
                             Thread.sleep(waitTime);
                         }
 
-                        req.setHeader("If-Modified-Since", lastModified);
+                        if (lastModified != null) {
+                            req.setHeader("If-Modified-Since", lastModified);
+                        }
 
                         Requester requester = req.withUrlPath(apiUrl);
                         GitHubResponse<GHThread[]> response = ((GitHubPageContentsIterable<GHThread>) requester

src/test/java/org/kohsuke/github/AppTest.java

@@ -1709,6 +1709,7 @@ public void notifications() throws Exception {
         }
         assertThat(found, is(true));
         gitHub.listNotifications().markAsRead();
+        gitHub.listNotifications().iterator().next();
     }
 
     /**

src/test/resources/org/kohsuke/github/AppTest/wiremock/notifications/__files/27-notifications.json

@@ -0,0 +1,5 @@
+{
+  "message": "Unable to parse If-Modified-Since request header. Please make sure value is in an acceptable format.",
+  "documentation_url": "https://docs.github.com/rest/activity/notifications#list-notifications-for-the-authenticated-user",
+  "status": "422"
+}

src/test/resources/org/kohsuke/github/AppTest/wiremock/notifications/mappings/26-notifications.json

@@ -40,8 +40,7 @@
       "X-XSS-Protection": "1; mode=block",
       "Referrer-Policy": "origin-when-cross-origin, strict-origin-when-cross-origin",
       "Content-Security-Policy": "default-src 'none'",
-      "X-GitHub-Request-Id": "CB13:833E:A1F759:BFE352:5DB3A151",
-      "Link": "<https://api.github.com/notifications?page=2>; rel=\"next\", <https://api.github.com/notifications?page=5>; rel=\"last\""
+      "X-GitHub-Request-Id": "CB13:833E:A1F759:BFE352:5DB3A151"
     }
   },
   "uuid": "ac22e3e2-f0d3-4ff1-af23-23e9c79c725c",

src/test/resources/org/kohsuke/github/AppTest/wiremock/notifications/mappings/27-notifications.json

@@ -0,0 +1,47 @@
+{
+  "id": "ac22e3e2-f0d3-4ff1-af23-23e9c7915874",
+  "name": "notifications",
+  "request": {
+    "url": "/notifications",
+    "method": "GET",
+    "headers": {
+      "Accept": {
+        "equalTo": "application/vnd.github+json"
+      },
+      "If-Modified-Since":{
+        "equalTo": "null"
+      }
+    }
+  },
+  "response": {
+    "status": 422,
+    "bodyFileName": "27-notifications.json",
+    "headers": {
+      "Date": "Wed, 20 Nov 2024 13:55:23 GMT",
+      "Content-Type": "application/json; charset=utf-8",
+      "X-OAuth-Scopes": "repo",
+      "X-Accepted-OAuth-Scopes": "notifications, repo",
+      "X-GitHub-Media-Type": "github.v3; format=json",
+      "x-github-api-version-selected": "2022-11-28",
+      "X-RateLimit-Limit": "5000",
+      "X-RateLimit-Remaining": "4992",
+      "X-RateLimit-Reset": "1732111985",
+      "X-RateLimit-Used": "8",
+      "X-RateLimit-Resource": "core",
+      "Access-Control-Expose-Headers": "ETag, Link, Location, Retry-After, X-GitHub-OTP, X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Used, X-RateLimit-Resource, X-RateLimit-Reset, X-OAuth-Scopes, X-Accepted-OAuth-Scopes, X-Poll-Interval, X-GitHub-Media-Type, X-GitHub-SSO, X-GitHub-Request-Id, Deprecation, Sunset",
+      "Access-Control-Allow-Origin": "*",
+      "Strict-Transport-Security": "max-age=31536000; includeSubdomains; preload",
+      "X-Frame-Options": "deny",
+      "X-Content-Type-Options": "nosniff",
+      "X-XSS-Protection": "0",
+      "Referrer-Policy": "origin-when-cross-origin, strict-origin-when-cross-origin",
+      "Content-Security-Policy": "default-src 'none'",
+      "Vary": "Accept-Encoding, Accept, X-Requested-With",
+      "Server": "github.com",
+      "X-GitHub-Request-Id": "052F:2E353B:3FE0C:42EBB:673DEA4B"
+    }
+  },
+  "uuid": "ac22e3e2-f0d3-4ff1-af23-23e9c7915874",
+  "persistent": true,
+  "insertionIndex": 26
+}