CHeapBitMap(MEMFLAGS flags) constructor misleading use of super-constructor

The `explicit CHeapBitMap(MEMFLAGS flags)` calls the `GrowableBitMap(bm_word_t* map, idx_t size_in_bits)` super-constructor with `GrowableBitMap(0, false)`. This works because 0 gets converted to nullptr and false gets converted to 0. However this seems to have be a miss in the most recent refactoring and is misleading.

 `explicit CHeapBitMap(MEMFLAGS flags)' should use `GrowableBitMap()` directly which creates an empty bitmap.