Do not reset the broadcast timer if there are no wants

[gammazero]

This change removes unnecessary timer activity and calls to broadcast.

Credit to @Wondertan for finding and proposing fix.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.44%. Comparing base (7489b98) to head (022bd45).
Report is 1 commits behind head on main.

@@            Coverage Diff             @@
##             main     #758      +/-   ##
==========================================
+ Coverage   60.40%   60.44%   +0.03%     
==========================================
  Files         245      245              
  Lines       31101    31102       +1     
==========================================
+ Hits        18787    18799      +12     
+ Misses      10641    10632       -9     
+ Partials     1673     1671       -2     

Files with missing lines	Coverage Δ
bitswap/client/internal/session/session.go	90.56% <100.00%> (+0.03%)	⬆️

... and 11 files with indirect coverage changes