[pxiels-core]: null values are likely not handled properly.

[bianhq]

Isnull is always false in column readers in writer.TestPixelsWriter.testRead() in pixels-core unit tests.
This bug may only happen when the row group is very small.

[bianhq]

The reason is found:
In io.pixelsdb.pixels.core.utils.BitUtils's two methods:
bitWiseDeCompact(byte[] bits, ByteBuffer input, int offset, int length)
and
bitWiseDeCompact(byte[] bits, ByteBuf input, int offset, int length),
the while loop condition while (offset < length) is incorrect.

This causes that bitWiseDeCompact does not execute the while loop body and leaves the bits unchanged.

This method is used by column readers to decompact isNull array from file, thus leads to incorrect isnull being parsed.

[bianhq]

This issue is not only 'likely' to happen, it is a 'must' happen.

[bianhq]

Resolved in PR #102.