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
```
def greet(name):
    if name==None:
        return None
    if name=='':
        return None
    n=len(name)    
    if n>=1:
        return "hello"+' '+name+'!'
def greet(name):
    if name==None:
        return None
    if name=='':
        return None
    n=len(name)    
    if n>=1:
        return "hello"+' '+name+'!'