fix lines resulting in string-bytes comparison error

[ftaebi]

Using python 3.8.3 I got an exception: "BytesWarning: Comparison between bytes and string", which is fixed by comparing with an empty bytestring instead of a normal empty string.

[sourcery-ai[bot]]

Sourcery Code Quality Report

✅  Merging this PR will increase code quality in the affected files by 0.03%.

Quality metrics	Before	After	Change
Complexity	2.81 ⭐	2.81 ⭐	0.00
Method Length	31.04 ⭐	30.92 ⭐	-0.12 👍
Working memory	5.50 ⭐	5.50 ⭐	0.00
Quality	84.18% ⭐	84.21% ⭐	0.03% 👍

Other metrics	Before	After	Change
Lines	124	124	0

Changed files	Quality Before	Quality After	Quality Change
pyxlsb/reader.py	84.18% ⭐	84.21% ⭐	0.03% 👍

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Working Memory	Quality	Recommendation
pyxlsb/reader.py	BIFF12Reader.next	9 🙂	113 🙂	11 😞	56.59% 🙂	Extract out complex expressions

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[willtrnr]

Thanks for that, I'll try to cut a patch release soon