message: processor: use empty file instead of <unknown> placeholder
diff: @@ -51,7 +51,7 @@ export default function processor(err: AirbrakeError, cb: Callback): void {
  for (let frame of frames) {
  backtrace.push({
  function: frame.functionName || '',
  - file: frame.fileName || '<unknown>',
  + file: frame.fileName || '',
  line: frame.lineNumber || 0,
  column: frame.columnNumber || 0,
  });
  