output warnings if nonsense commas are included

Author: yuheiy

src/index.ts

@@ -80,6 +80,7 @@ const creator: PluginCreator<pluginOptions> = (
 
 						// contains commas without meaningful tokens, such as `--negative(,)`
 						if (nodes.length === 0) {
+							decl.warn(result, 'Syntax error');
 							return componentValue;
 						}
 

test/_tape.js

@@ -13,5 +13,6 @@ postcssTape(plugin)({
 				'--comma-separated-list-of-component-values': () => '10px, 20px',
 			},
 		},
+		warnings: 3,
 	},
 });