When setting keys in ECB mode, don't check IV length.

[smaxa20]

I was trying to encrypt a string using a generated key and no IV because I'm using ECB. Here's my code:

var crypt = AesCrypt();
crypt.aesSetMode(AesMode.ecb);
Uint8List key = crypt.createKey();
crypt.aesSetKeys(key);
...

When I go to set the key it fails giving this error:
Unhandled Exception: NoSuchMethodError: The getter 'length' was called on null.

With this partial stack trace:

E/flutter ( 7509): #1      _Aes.aesSetKeys          package:aes_crypt/src/aes.dart:199
E/flutter ( 7509): #2      AesCrypt.aesSetKeys      package:aes_crypt/src
...

Following this trace to aes_crypt/src/aes.dart:199 I find this code:

aes_crypt/lib/src/aes.dart

Line 199 in 68cb8f7

} else if (iv.length != 16) {

It checks the length of the IV even in ECB mode when ECB mode doesn't require an IV (if my understanding is correct).
I've made this check only occur when the mode is not ECB.