Use [].flat instead .reduce + .concat to avoid error on empty arrays.

When piplelinesToSave is an empty array [] line 231 will throw `TypeError: Reduce of empty array with no initial value`. We could add a second argument to .reduce, but .flat does what we're doing with .reduce + .concat and avoid the error on empty arrays

x-pack/legacy/plugins/epm/server/datasources/create.ts

@@ -228,7 +228,7 @@ async function installAssets({
   // the promises from template installation resolve to template references
   const templatesToSave = await Promise.all(templateRefs);
   // the promises from pipeline installation resolve to arrays of pipeline references
-  const pipelinesToSave = (await Promise.all(pipelineRefs)).reduce((a, b) => a.concat(b));
+  const pipelinesToSave = (await Promise.all(pipelineRefs)).flat();
 
   return templatesToSave.concat(pipelinesToSave);
 }