fix empty details

lib/winston-loggly.js

@@ -131,12 +131,13 @@ Loggly.prototype.log = function(meta, callback) {
 
   const splats = data[Symbol.for('splat')];
   if (splats && splats.length > 0) {
-    data.details =
+    let details =
       typeof splats[0] === 'object' ? splats.slice(1, splats.length) : splats; //ignore the first object, it's already included in root
     if (splats[0].details !== undefined) {
       //overwrites the original details prop, so we need to include it
-      data.details = [{ details: splats[0].details }, ...data.details];
+      details = [{ details: splats[0].details }, ...data.details];
     }
+    if (details.length) data.details = details;
   }
 
   const self = this;