Unable to create pulsar function with --window-length-duration-ms or --window-length-count

**Describe the bug**

There are two ways to develop a custom pulsar function. 1.) by implementing interface Function 2.) by implementing WindowFunction.

Custom pulsar function that is developed using interface Function has a null issue When we try to create function with options --window-length-duration-ms or --window-length-count.
This issue observed in Pulsar version:2.6.1 and Pulsar version-all :2.6.1

bin/pulsar-admin functions create \--jar test.jar --classname com.abcd.xyz.mnop.qrst.function.sample --inputs input --output test --name samplefunction --window-length-count 30000
null

Reason: null

**To Reproduce**
Steps to reproduce the behavior:

1. Develop a custom pulsar function by implementing interface Function
```
public class sample implements Function {

@Override
public Void process(String input, Context context) throws Exception {
System.out.println("Test:"+input);
return null;
}
}
```
2. Generate the jar (sample.jar )for this custom pulsar function
3. Deploy the generated jar (sample.jar) in pulsar container by docker cp sample.jar :/pulsar
4. login/run the container by docker exec -it /bin/bash
5. Now execute the following command bin/pulsar-admin functions create \--jar sample.jar --classname com.abcd.xyz.mnop.qrst.function.sample --inputs input --output test --name samplefunction --window-length-count 300
Following error occur


**Expected behavior**
Pulsar function should be created with --window-length-duration-ms and --window-length-count options

**Screenshots**
NA

**Desktop (please complete the following information):**
- OS: Windows 10, Pulsar in Docker image. Docker version: 2.2.0.4

**Additional context**
NA

Start by reproducing the issue with the sample Java Function implementation and the bin/pulsar-admin functions create command using --window-length-count or --window-length-duration-ms in the Docker-based Pulsar setup. Trace the command's handling of these options and compare it with WindowFunction creation; done means a Function can be created successfully with either option instead of returning null.

Written by the indexing model from the issue text.