fix(navbar): handle products without a purchaseUrl

ProductLearnMoreLink crashed SSG for any product lacking a purchaseUrl:
the product matched in config but baseHref was undefined, so addUtmParams
called undefined.includes() and failed the build. Query Filters (a library
with no product page) was the first such product.

Now: render no commercial CTA for products without a purchase page, and
guard addUtmParams against a falsy URL (defense in depth).

Author: zackkatz

src/theme/NavbarItem/ProductLearnMoreLink.js

@@ -20,6 +20,9 @@ const defaultLink = {
 
 // Add UTM parameters to URL
 function addUtmParams(url, productId) {
+  if (!url) {
+    return url;
+  }
   const utmParams = new URLSearchParams({
     utm_source: 'developer-docs',
     utm_medium: 'navbar',
@@ -40,6 +43,14 @@ export default function ProductLearnMoreLink() {
   // Get product config or use default
   const product = productId && productConfig[productId];
 
+  // A product may exist in config without a purchaseUrl (e.g. a developer
+  // library with no product page). It has no commercial "learn more"
+  // destination, so render nothing rather than crash on an undefined URL.
+  const isProductWithoutPurchaseUrl = product && !product.purchaseUrl;
+  if (isProductWithoutPurchaseUrl) {
+    return null;
+  }
+
   const linkText = product ? `Learn more about ${product.label}` : defaultLink.label;
   const baseHref = product ? product.purchaseUrl : defaultLink.href;
   const linkHref = addUtmParams(baseHref, productId);