ENH: An argument for .query() that returns all rows
#### Is your feature request related to a problem?

Pandas query strings are very useful, however when using a variable to pass in the query, e.g.

```df.query(my_query)```

There is no null or empty `my_query` that will return all rows. An empty string returns a ValueError. Any non-string argument returns an error.

#### Describe the solution you'd like

An empty string or None or some keyword argument (even with an empty `expr` to .query() should return all rows.

#### Additional context

Without the ability to pass something to `.query` that returns all rows you end up with something like this, when `_query` may or may not set. It'd be nice to be able to avoid this conditional by having something to pass to query that returns all rows.

```python

if _query:
   dfs = [f(x).query(_query) for x in the_list]
else:
   dfs = [f(x) for x in list]

```
