Fix handling nil values for optional string array parameters,

pkg/github/issues_test.go

@@ -468,9 +468,10 @@
 				),
 			),
 			requestArgs: map[string]interface{}{
 				"owner": "owner",
 				"repo":  "repo",
 				"title": "Minimal Issue",
+				"assignees": nil,	// Expect no failure with nil optional value.
 			},
 			expectError: false,
 			expectedIssue: &github.Issue{

pkg/github/server.go

@@ -232,12 +232,14 @@
 // 1. Checks if the parameter is present in the request, if not, it returns its zero-value
 // 2. If it is present, iterates the elements and checks each is a string
 func OptionalStringArrayParam(r mcp.CallToolRequest, p string) ([]string, error) {
-	// Check if the parameter is present in the request
+    // Check if the parameter is present in the request
 	if _, ok := r.Params.Arguments[p]; !ok {
 		return []string{}, nil
 	}
 
-	switch v := r.Params.Arguments[p].(type) {
+    switch v := r.Params.Arguments[p].(type) {
+	case nil:
+		return []string{}, nil
 	case []string:
 		return v, nil
 	case []any: