Handle DEFAULT table access name in ALTER TABLE

[mkindahl]

If ALTER TABLE ... SET ACCESS METHOD DEFAULT is used, the name of the table access method is NULL in the AlterTableCmd, so add checks that the name is not null before using it.

Function hypercore_alter_access_method_finish expects a chunk, but could be called with a non-chunk, so checking that the relid is a chunk before calling it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.85%. Comparing base (59f50f2) to head (53d1bb0).
Report is 806 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7799      +/-   ##
==========================================
+ Coverage   80.06%   81.85%   +1.78%     
==========================================
  Files         190      247      +57     
  Lines       37181    45462    +8281     
  Branches     9450    11371    +1921     
==========================================
+ Hits        29770    37212    +7442     
- Misses       2997     3766     +769     
- Partials     4414     4484      +70     

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[erimatnor]

Approving, although I think you can make one small change to improve.

[timescale-automation]

Automated backport to 2.18.x not done: cherry-pick failed.

Git status

HEAD detached at origin/2.18.x
You are currently cherry-picking commit 32ec872d5.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	new file:   .unreleased/pr_7799
	modified:   src/process_utility.c
	new file:   test/expected/tableam_alter_defaults.out
	modified:   test/sql/CMakeLists.txt
	new file:   test/sql/tableam_alter_defaults.sql
	modified:   tsl/src/process_utility.c

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tsl/src/hypercore/hypercore_handler.c

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