fix(react-table): check empty array in areAllRowsSelected

[priley86]

What:
If an empty array is passed for rows (along with onSelect for the "Selectable table" variation), currently the "all rows selection" checkbox in the top left will be set true b/c rows.every evaluates true with an empty array.

const isSelected = row => row.selected === true; 
let rowsArray = []; 
rowsArray.every(item => isSelected(row));

returns true when called.

Noted this while testing selection downstream today...

Additional issues:

[patternfly-build]

PatternFly-React preview: https://patternfly-react-pr-2527.surge.sh

[redallen]

There's probably an isArray() type deal we can use in case we get passed a weird object with a length prop that isn't an array, but this will suffice.