test(bytesconv): add tests for empty/nil cases

[miladev95]

Added two small tests to internal/bytesconv/bytesconv_test.go:

TestBytesToStringEmpty — asserts BytesToString([]byte{}) and BytesToString(nil) return "".
TestStringToBytesEmpty — asserts StringToBytes("") returns an empty []byte. Verified with go test ./internal/bytesconv -v.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.91%. Comparing base (3dc1cd6) to head (ddb67d1).
⚠️ Report is 219 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4454      +/-   ##
==========================================
- Coverage   99.21%   98.91%   -0.30%     
==========================================
  Files          42       44       +2     
  Lines        3182     2959     -223     
==========================================
- Hits         3157     2927     -230     
- Misses         17       19       +2     
- Partials        8       13       +5     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.91% <ø> (?)
-tags go_json	98.77% <ø> (?)
-tags nomsgpack	98.83% <ø> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.91% <ø> (?)
go-1.25	98.85% <ø> (?)
macos-latest	98.85% <ø> (-0.37%)	⬇️
ubuntu-latest	98.91% <ø> (-0.30%)	⬇️

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