fix(NODE-5355): prevent error when saslprep is not a function (#3733)

Author: baileympearson

lib/core/auth/scram.js

@@ -25,7 +25,7 @@ class ScramSHA extends AuthProvider {
 
   prepare(handshakeDoc, authContext, callback) {
     const cryptoMethod = this.cryptoMethod;
-    if (cryptoMethod === 'sha256' && saslprep == null) {
+    if (cryptoMethod === 'sha256' && typeof saslprep !== 'function') {
       emitWarningOnce('Warning: no saslprep library specified. Passwords will not be sanitized');
     }
 
@@ -125,7 +125,7 @@ function continueScramConversation(cryptoMethod, response, authContext, callback
 
   let processedPassword;
   if (cryptoMethod === 'sha256') {
-    processedPassword = saslprep ? saslprep(password) : password;
+    processedPassword = typeof saslprep === 'function' ? saslprep(password) : password;
   } else {
     try {
       processedPassword = passwordDigest(username, password);