fix: make explicit data request when clearing cache with no initial pool

[yuriy-fix]

Fixes vaadin/flow-components#427

[tomivirkki]

Did some debugging with the original issue and this might be fixable in the data-provider-mixin:

Seems that the <tbody> is empty, so initial pool has never been created. Added a debugger to increasePoolIfNeeded...

It was never called...

...so added another one before the guard statement. It got caught a couple of times before the panel was opened, but also once after it got opened. All the other conditions were fine but canPopulate() was returning false

The canPopulate() function checks for _hasData and _columnTree and out of those two, it was the _hasData that was falsy...

Seems that _hasData gets set true when a data provider responds...

...so it must have been true at some point.

The only place that sets it false is clearCache(), and sort-mixin indeed calls clearCache() (once the sorters are set).

The end of clearCache() makes an explicit data request, but only in the case the grid doesn't have physical items/rows/effectiveSize beforehand. If it does, then it relies on the grid rows to ask for more data and the explicit data request is skipped.

But the condition here seems to be wrong since even though the grid has _effecticeSize, there are 0 physical rows rendered at this point and thus the new data request never gets made (and _hasData will remain false).

So one fix might be to change the condition to also make the explicit data request in case the initial pool hasn't been created:

The PR's original revision works because it explicitly calls createPool right before the sort-mixin's clearCache manages to change _hasData to false. It's probably a plausible fix in case sorters are used but an explicit call to clearCache() of a hidden grid might still suffer from the same issue (that's probably a good scenario for a test case for the fix). If possible, it's better to fix the root cause of the issues.

[web-padawan]

Just a few comments related to the code style.