Fix bug for merged documents not having source

Sources/GraphQLCompiler/GraphQLError.swift

@@ -59,7 +59,7 @@ public final class GraphQLError: JavaScriptError {
   /// See https://shazronatadobe.wordpress.com/2010/12/04/xcode-shell-build-phase-reporting-of-errors/
   public var logLines: [String]? {
     return sourceLocations?.map {
-      return [$0.filePath, String($0.lineNumber), "error", message ?? "?"].joined(separator: ":")
+      return [$0.filePath ?? "", String($0.lineNumber), "error", message ?? "?"].joined(separator: ":")
     }
   }
 }

Sources/GraphQLCompiler/GraphQLJSFrontend.swift

@@ -73,8 +73,9 @@ public final class GraphQLJSFrontend {
     from fileURL: URL,
     experimentalClientControlledNullability: Bool = false
   ) async throws -> GraphQLDocument {
+    let source = try await makeSource(from: fileURL)
     return try await parseDocument(
-      try makeSource(from: fileURL),
+      source,
       experimentalClientControlledNullability: experimentalClientControlledNullability
     )
   }

Sources/GraphQLCompiler/GraphQLSource.swift

@@ -18,7 +18,7 @@ public final class GraphQLSource: JavaScriptObject {
 
 /// Represents a location in a GraphQL source file.
 public struct GraphQLSourceLocation {
-  let filePath: String
+  let filePath: String?
 
   let lineNumber: Int
   let columnNumber: Int
@@ -35,9 +35,9 @@ public struct GraphQLSourceLocation {
 public class ASTNode: JavaScriptObject, @unchecked Sendable {
   public let kind: String
 
-  public let source: GraphQLSource
+  public let source: GraphQLSource?
 
-  public var filePath: String { source.filePath }
+  public var filePath: String? { source?.filePath }
 
   required init(_ jsValue: JSValue, bridge: isolated JavaScriptBridge) {
     self.kind = jsValue["kind"]