releasetools: Fix a bug in blockimgdiff.HeapItem.

HeapItem defines __bool__(), which contains a logical error that should
return the opposite value.

Note that the bug only manifests while using Python 3, which calls
__bool__(). With Python 2, `if x:` or bool(x) actually calls
x.__nonzero__() or x.__len__(). If a class defines neither __len__() nor
__nonzero__(), as the case in HeapItem, it always returns True.

Test: python -m unittest test_blockimgdiff
Test: python3 -m unittest test_blockimgdiff
Test: Generate an incremental non-A/B OTA package successfully.
Change-Id: Ibe8430e0b495a7d2f430cfffb716d2536ffb53d2