supply a default link.label array, as the hover tooltip offsets text if a non-string falsey value is provided

Author: monfera

src/traces/sankey/defaults.js

@@ -31,13 +31,13 @@ module.exports = function supplyDefaults(traceIn, traceOut, defaultColor, layout
         return Color.addOpacity(defaultNodePalette(i), 0.8);
     }));
 
-    coerce('link.label');
     coerce('link.source');
     coerce('link.target');
     coerce('link.value');
     coerce('link.line.color');
     coerce('link.line.width');
 
+    coerce('link.label', traceOut.link.value.map(function() {return '';}));
     coerce('link.color', traceOut.link.value.map(function() {
         return tinycolor(layout.paper_bgcolor).getLuminance() < 0.333 ?
             'rgba(255, 255, 255, 0.6)' :