setKeystoreFile for a resource file in class path

org.apache.ftpserver.ssl.SslConfigurationFactory#loadStore support file or resource.

but org.apache.ftpserver.ssl.SslConfigurationFactory#setKeystoreFile only support file , it will check file length:

```
public void setKeystoreFile(File keyStoreFile) {
if (keyStoreFile != null && keyStoreFile.length() != 0L) {
this.keystoreFile = keyStoreFile;
} else {
throw new FtpServerConfigurationException("KeystoreFile must not be null or zero length");
}
}

```
resolve this is to remove the check ：

```
public void setKeystoreFile(File keyStoreFile) {

this.keystoreFile = keyStoreFile;

}
```