Fix col.names="none" suppressing all output when row.names=FALSE

[venom1204]

closes #7735

the PR fixes a bug where setting col.names="none" combined with row.names=FALSE resulted in zero output.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.85%. Comparing base (deb9acc) to head (9c8500a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7804   +/-   ##
=======================================
  Coverage   98.85%   98.85%           
=======================================
  Files          87       87           
  Lines       17140    17147    +7     
=======================================
+ Hits        16944    16951    +7     
  Misses        196      196           

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[github-actions]

• HEAD=issue773355 much slower for as.data.table.array improved in #7019
• HEAD=issue773355 much slower for transform improved in #5493
• HEAD=issue773355 slower P<0.001 for DT[by] max regression fixed in #7480
• HEAD=issue773355 faster P<0.001 for memrecycle regression fixed in #5463
  

Generated via commit 9c8500a

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	6 minutes and 26 seconds
Installing different package versions	11 minutes and 44 seconds
Running and plotting the test cases	5 minutes and 29 seconds

[tdhock]

thanks.
please add NEWs item, and add PR comment which explains behavior before/after this PR?

[tdhock]

please revise