[misc] enhance TaskRunner Process

[Irving-cl]

This PR adds a check in TaskRunner::Process so that 'read' is only executed when the eventFd is set.

Currently if we use strace to see the i/o of otbr-agent, we can see many things like this:

read(7, 0x7ffc7c0f80ff, 1)              = -1 EAGAIN (Resource temporarily unavailable)
read(9, 0x7ffc7c0f80ff, 1)              = -1 EAGAIN (Resource temporarily unavailable)
read(4, 0x7ffc7c0f80ff, 1)              = -1 EAGAIN (Resource temporarily unavailable)
read(7, 0x7ffc7c0f80ff, 1)              = -1 EAGAIN (Resource temporarily unavailable)
read(9, 0x7ffc7c0f80ff, 1)              = -1 EAGAIN (Resource temporarily unavailable)
read(4, 0x7ffc7c0f80ff, 1)              = -1 EAGAIN (Resource temporarily unavailable)

Though this is harmless, we should avoid it as possible for easier debugging and reduce overhead.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 43.29%. Comparing base (2b41187) to head (9d63ccd).
Report is 938 commits behind head on main.

Files with missing lines	Patch %	Lines
src/common/task_runner.cpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2697       +/-   ##
===========================================
- Coverage   55.77%   43.29%   -12.48%     
===========================================
  Files          87      108       +21     
  Lines        6890    13400     +6510     
  Branches        0      963      +963     
===========================================
+ Hits         3843     5802     +1959     
- Misses       3047     7291     +4244     
- Partials        0      307      +307     

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[superwhd]

LGTM 👍

Thanks for fixing this!