fix Bound filter's empty argument checking for 'lower' and 'upper'

[hzhu212]

Hi guys, I found a quite obvious bug when I use the Bound filter.

The minimum code to reproduce the bug is as below:

from pydruid.utils.filters import Bound

# I want to select the rows that satisfy "foo >= 0"
Bound(dimension='foo', lower=0).show()

this will cause the following exception:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-76-afbba87ff9ee> in <module>
      1 from pydruid.utils.filters import Bound
----> 2 Bound(dimension='foo', lower=0).show()

~/Library/Python/3.7/lib/python/site-packages/pydruid/utils/filters.py in __init__(self, dimension, lower, upper, lowerStrict, upperStrict, alphaNumeric, ordering, extraction_function)
    216     ):
    217         if not lower and not upper:
--> 218             raise ValueError("Must include either lower or upper or both")
    219         Filter.__init__(
    220             self,

ValueError: Must include either lower or upper or both

But when I changed lower=0 into lower=1, everything just went well:

Bound(dimension='foo', lower=1).show()

output:

{
    "filter": {
        "type": "bound",
        "dimension": "foo",
        "lower": 1,
        "lowerStrict": false,
        "upper": null,
        "upperStrict": false,
        "alphaNumeric": false,
        "ordering": "lexicographic"
    }
}

Obviously, the root of the problem lies in line 217, which is to check whether argument lower and upper are both omitted:

if not lower and not upper:

However, this checking unexpectedly blocked the number 0, besides None.

Changing this line into the line below should fix the bug:

if lower is None and upper is None:

[pranatishete]

@hzhu212 This property can be overcome if you pass ordering="numeric" and pass lower and upperbound as 0