Fix handling nil values for optional string array parameters,

nil values should be equivalent to an empty string, currently we return an error but Claude passes nil for optional values.

Author: A-Georgiou

pkg/github/issues_test.go

@@ -471,6 +471,7 @@ func Test_CreateIssue(t *testing.T) {
 				"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 @@ func OptionalIntParamWithDefault(r mcp.CallToolRequest, p string, d int) (int, e
 // 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: