[Proposal] Add functions `remove_empty_fields` `remove_null_fields`

Proposal for adding built-in functions `remove_empty_fields` and `remove_null_fields` which would be useful to remove fields of an object that are empty or have null value.

e.g. if we have a json like this

```
{'a':1, 'b':'', 'c': null}
```

we call

```
remove_empty_fields(@)
```

Result
```
{'a':1}
```

if we call

```
remove_null_fields(@)
```

Result
```
{'a':1, 'b':''}
```