Clarify semantic tag normalization

Author: molang163

src/lib/semantic-tags.ts

@@ -7,12 +7,13 @@ import { getW3CDateString } from './w3cdate';
 function normalizeSemanticValue(value: SemanticTagValue): SemanticTagValue {
   if (value instanceof Date) {
     if (!Number.isFinite(value.getTime()))
-      throw new TypeError(`Semantic tag dates must be valid Date instances`);
+      throw new TypeError(`Semantic tag Date values must be valid`);
     return getW3CDateString(value);
   }
 
   if (Array.isArray(value)) return value.map(normalizeSemanticValue);
 
+  // The truthiness check excludes null, because typeof null is 'object'.
   if (value && typeof value === 'object') {
     const result: { [key: string]: SemanticTagValue } = {};
     for (const [key, nestedValue] of Object.entries(value))