fix(fork): Validate that name follows GitHub repository naming conven…

…tions and add @nullable annotations for optional parameters

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

@@ -1501,24 +1501,23 @@ public GHRepository forkTo(GHOrganization org) throws IOException {
     /**
      * Creates a fork of this repository with optional parameters.
      *
-     * @param organization
-     *            the organization to fork to, or null to fork to the authenticated user's account
-     * @param name
-     *            the name of the new repository, or null to use the same name as the original repository
-     * @param defaultBranchOnly
-     *            whether to fork only the default branch
+     * @param organization      the organization to fork to, or null to fork to the authenticated user's account
+     * @param name              the name of the new repository, or null to use the same name as the original repository
+     * @param defaultBranchOnly whether to fork only the default branch
      * @return the newly forked repository
-     * @throws IOException
-     *             if an I/O error occurs
+     * @throws IOException if an I/O error occurs
      */
-    public GHRepository createFork(String organization, String name, boolean defaultBranchOnly) throws IOException {
+    public GHRepository createFork(@Nullable String organization, @Nullable String name, boolean defaultBranchOnly) throws IOException {
+
         if (organization != null && organization.isEmpty()) {
             throw new IllegalArgumentException("Organization cannot be empty");
         }
         if (name != null && name.isEmpty()) {
             throw new IllegalArgumentException("Name cannot be empty");
         }
-
+        if (name != null && !name.matches("^[a-zA-Z0-9._-]+$")) {
+            throw new IllegalArgumentException("Repository name contains invalid characters");
+        }
         Requester requester = root().createRequest().method("POST").withUrlPath(getApiTailUrl("forks"));
 
         if (organization != null) {