chore: Server.new_session: Create session w/o name

[tony]

Fixes #399

[codecov]

Codecov Report

Merging #400 (563354f) into master (3e08aad) will increase coverage by 0.03%.
The diff coverage is 66.66%.

❗ Current head 563354f differs from pull request most recent head 0dfdc5b. Consider uploading reports for the commit 0dfdc5b to get more accurate results

@@            Coverage Diff             @@
##           master     #400      +/-   ##
==========================================
+ Coverage   88.54%   88.57%   +0.03%     
==========================================
  Files          18       18              
  Lines        1851     1856       +5     
==========================================
+ Hits         1639     1644       +5     
  Misses        212      212              

Impacted Files	Coverage Δ
libtmux/server.py	80.66% <42.85%> (ø)
tests/test_server.py	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more