Solve the programming task below in a Python markdown code block.
Say hello!

Write a function to greet a person. Function will take name as input and greet the person by saying hello.
Return null/nil/None if input is empty string or null/nil/None.

Example: 

```python
greet("Niks") --> "hello Niks!"
greet("")    --> None # Return None if input is empty string
greet(None)  --> None # Return None if input is None
```
Also feel free to reuse/extend the following starter code:
```python
def greet(name):
```