Log httpServer.close error only when defined

[sbcgua]

Summary

fix #1368
Using socketMode = true with customRoutes issues irrelevant error message on app.stop

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #1369 (fc4e5b8) into main (4288c1e) will decrease coverage by 0.06%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #1369      +/-   ##
==========================================
- Coverage   72.76%   72.69%   -0.07%     
==========================================
  Files          17       17              
  Lines        1465     1465              
  Branches      436      437       +1     
==========================================
- Hits         1066     1065       -1     
  Misses        310      310              
- Partials       89       90       +1     

Impacted Files	Coverage Δ
src/receivers/SocketModeReceiver.ts	87.64% <0.00%> (-1.13%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4288c1e...fc4e5b8. Read the comment docs.

[srajiang]

Thanks for catching this edge-case @sbcgua and thank you for the contribution. LGTM!