Fix writing with no termination character

[BT8298]

Previously, if the write termination was the empty byte, the written message was truncated to the empty byte.

I am fairly certain that serial (not GPIB, where end of input signal exists) instruments that are compliant with IEEE 488.2 must have a linefeed as a write termination (§ 7.5.2 of IEEE 488.2-1992(R2004)). Some non-compliant instruments also have write terminations, like modems using the Hayes AT command set. But instruments without a write termination character exist. Thorlabs motors using the APT protocol come to mind.

[codecov]

Codecov Report

❌ Patch coverage is 95.31250% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.50%. Comparing base (aaddb17) to head (26ccc73).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pyvisa_sim/devices.py	93.33%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
+ Coverage   86.64%   87.50%   +0.85%     
==========================================
  Files          18       19       +1     
  Lines        1041     1104      +63     
  Branches      113      124      +11     
==========================================
+ Hits          902      966      +64     
+ Misses        104      103       -1     
  Partials       35       35              

Flag	Coverage Δ
unittests	87.50% <95.31%> (+0.85%)	⬆️

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[BT8298]

Thoughts on this test failing? I think it's fine if this one fails, since instruments with no write termination character implement traffic control through mechanisms like fixed message length or check-summing. There are no SCPI or IEEE 488.2 serial devices without write termination to my knowledge, so this test case does not represent any real instrument.