message: fix(ember): Add guards to ensure marks exist
  This adds some guards before performing the initial load performance.measure to ensure that the performance marks exist. If the marks don't exist an error is thrown.
diff: @@ -305,6 +305,12 @@ function _instrumentInitialLoad(config: EmberSentryConfig) {
  }
  const measureName = '@sentry/ember:initial-load';
  + const startMarkExists = performance.getEntriesByName(startName).length > 0;
  + const endMarkExists = performance.getEntriesByName(endName).length > 0;
  + if (!startMarkExists || !endMarkExists) {
  + return;
  + }
  +
  performance.measure(measureName, startName, endName);
  const measures = performance.getEntriesByName(measureName);
  const measure = measures[0];
  