fix: reading an unset style property returns "" per CSSOM, not undefined

cjs/interface/css-style-declaration.js

@@ -36,7 +36,10 @@ const handler = {
       return getKeys(style).length;
     if (/^\d+$/.test(name))
       return getKeys(style)[name];
-    return style.get(uhyphen(name));
+    // Per CSSOM, reading a property that is not set returns the empty string,
+    // not undefined (which also matches getPropertyValue, routed here).
+    const value = style.get(uhyphen(name));
+    return value === void 0 ? '' : value;
   },
 
   set(style, name, value) {

esm/interface/css-style-declaration.js

@@ -35,7 +35,10 @@ const handler = {
       return getKeys(style).length;
     if (/^\d+$/.test(name))
       return getKeys(style)[name];
-    return style.get(uhyphen(name));
+    // Per CSSOM, reading a property that is not set returns the empty string,
+    // not undefined (which also matches getPropertyValue, routed here).
+    const value = style.get(uhyphen(name));
+    return value === void 0 ? '' : value;
   },
 
   set(style, name, value) {

test/interface/css-style-declaration.js

@@ -6,6 +6,10 @@ const {document} = parseHTML('');
 
 let node = document.createElement('div');
 assert(node.style.cssText, '', 'empty style');
+// Per CSSOM, an unset property reads as the empty string, not undefined —
+// matching getPropertyValue, which is routed through the same getter.
+assert(node.style.color, '', 'unset property getter returns ""');
+assert(node.style.getPropertyValue('color'), '', 'unset getPropertyValue returns ""');
 node.style.cssText = 'background-color: blue; background-image: url("https://t.co/i.png");';
 assert(node.style.backgroundColor, 'blue', 'style getter');
 assert(node.style.backgroundImage, 'url("https://t.co/i.png")', 'style value with colon');