Use another compound assignment statement

[elfring]

👀 Some source code analysis tools can help to find opportunities for improving software components.
💭 I propose to increase the usage of compound operators accordingly.

diff --git a/togeojson.js b/togeojson.js
index 23ec6cc..f006622 100644
--- a/togeojson.js
+++ b/togeojson.js
@@ -136,7 +136,7 @@ var toGeoJSON = (function() {
             }
             function kmlColor(v) {
                 var color, opacity;
-                v = v || '';
+                v ||= '';
                 if (v.substr(0, 1) === '#') { v = v.substr(1); }
                 if (v.length === 6 || v.length === 3) { color = v; }
                 if (v.length === 8) {