fix: Don't error on 'null' in HTML templates (#5602)

* fix: Don't render 'null' in HTML templates

* Add test for null handling

* Add changeset for html package

.changeset/fresh-months-tap.md

@@ -0,0 +1,5 @@
+---
+'@prairielearn/html': patch
+---
+
+Fix rendering of null values in templates

packages/html/src/index.test.ts

@@ -39,6 +39,10 @@ describe('html', () => {
   it('omits boolean values from template', () => {
     assert.equal(html`<p>${true}${false}</p>`.toString(), '<p></p>');
   });
+
+  it('omits nullish values from template', () => {
+    assert.equal(html`<p>${null}${undefined}</p>`.toString(), '<p></p>');
+  });
 });
 
 describe('escapeHtml', () => {

packages/html/src/index.ts

@@ -9,10 +9,13 @@ function escapeValue(value: unknown): string {
     return value.map((val) => escapeValue(val)).join('');
   } else if (typeof value === 'string' || typeof value === 'number') {
     return ejs.escapeXML(String(value));
+  } else if (value == null) {
+    // undefined or null -- render nothing
+    return '';
   } else if (typeof value === 'object') {
-    throw new Error('Cannot interpolate object in template');
+    throw new Error(`Cannot interpolate object in template: ${JSON.stringify(value)}`);
   } else {
-    // This is undefined, null, or a boolean - don't render anything here.
+    // This is boolean - don't render anything here.
     return '';
   }
 }