tls: permit null as a pfx value

Allow null along with undefined for pfx value.

This is to avoid breaking change when upgrading v14 to v16 and
3rd party library passing null to pfx

Fixes: nodejs#36292

lib/internal/tls/secure-context.js

@@ -261,7 +261,7 @@ function configSecureContext(context, options = {}, name = 'options') {
     context.setSessionIdContext(sessionIdContext);
   }
 
-  if (pfx !== undefined) {
+  if (pfx != null) {
     if (ArrayIsArray(pfx)) {
       ArrayPrototypeForEach(pfx, (val) => {
         const raw = val.buf ? val.buf : val;

test/parallel/test-tls-connect-secure-context.js

@@ -23,3 +23,20 @@ connect({
   assert.ifError(err);
   return cleanup();
 });
+
+connect({
+  client: {
+    servername: 'agent1',
+    secureContext: tls.createSecureContext({
+      ca: keys.agent1.ca,
+      pfx: null,
+    }),
+  },
+  server: {
+    cert: keys.agent1.cert,
+    key: keys.agent1.key,
+  },
+}, function(err, pair, cleanup) {
+  assert.ifError(err);
+  return cleanup();
+});