[Fix] stringify: apply formatter to encoded key under strictNullHandling

Previously the null branch returned the raw `encoder(prefix, ...)` output
(or raw `prefix`) without running it through `formatter`.
With `format: 'RFC1738'` this caused keys containing spaces to serialize as
`%20` in strict-null mode while the normal primitive path produced `+`,
yielding inconsistent output for the same key.

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: DeepView Autofix <276251120+deepview-autofix@users.noreply.github.com>
Co-Authored-By: Nikita Skovoroda <chalkerx@gmail.com>
Co-Authored-By: Jordan Harband <ljharb@gmail.com>
Signed-off-by: Nikita Skovoroda <chalkerx@gmail.com>

Author: 4 people

lib/stringify.js

@@ -118,7 +118,7 @@ var stringify = function stringify(
 
     if (obj === null) {
         if (strictNullHandling) {
-            return encoder && !encodeValuesOnly ? encoder(prefix, defaults.encoder, charset, 'key', format) : prefix;
+            return formatter(encoder && !encodeValuesOnly ? encoder(prefix, defaults.encoder, charset, 'key', format) : prefix);
         }
 
         obj = '';

test/stringify.js

@@ -1140,6 +1140,28 @@ test('stringify()', function (t) {
         st.end();
     });
 
+    t.test('strictNullHandling: applies the formatter to the encoded key (RFC1738)', function (st) {
+        st.equal(
+            qs.stringify(
+                { 'a b': null, 'c d': 'e f' },
+                { strictNullHandling: false, format: 'RFC1738' }
+            ),
+            'a+b=&c+d=e+f',
+            'without: as expected'
+        );
+
+        st.equal(
+            qs.stringify(
+                { 'a b': null, 'c d': 'e f' },
+                { strictNullHandling: true, format: 'RFC1738' }
+            ),
+            'a+b&c+d=e+f',
+            'with: as expected'
+        );
+
+        st.end();
+    });
+
     t.test('throws if an invalid charset is specified', function (st) {
         st['throws'](function () {
             qs.stringify({ a: 'b' }, { charset: 'foobar' });