Fix segfault in cagg creation

[svenklemm]

When trying to create cagg on top of any relation that is a neither a hypertable nor a continuous aggregate the command would segfault. This patch changes the code to handle this case gracefully and error out when trying to create a cagg on top of a relation that is not supported. Found by coverity.

[codecov]

Codecov Report

Merging #5031 (47e7741) into main (35c9120) will increase coverage by 0.06%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##             main    #5031      +/-   ##
==========================================
+ Coverage   89.65%   89.72%   +0.06%     
==========================================
  Files         227      227              
  Lines       51600    51507      -93     
==========================================
- Hits        46264    46214      -50     
+ Misses       5336     5293      -43     

Impacted Files	Coverage Δ
tsl/src/continuous_aggs/create.c	87.90% <90.00%> (-0.05%)	⬇️
src/loader/bgw_launcher.c	89.51% <0.00%> (-2.55%)	⬇️
src/bgw/scheduler.c	83.41% <0.00%> (-2.01%)	⬇️
src/loader/bgw_message_queue.c	88.63% <0.00%> (-0.57%)	⬇️
tsl/src/debug.c	49.36% <0.00%> (-0.33%)	⬇️
tsl/src/compression/api.c	95.57% <0.00%> (-0.25%)	⬇️
tsl/src/nodes/data_node_dispatch.c	93.03% <0.00%> (-0.22%)	⬇️
tsl/src/nodes/decompress_chunk/decompress_chunk.c	94.91% <0.00%> (-0.20%)	⬇️
tsl/test/src/test_ddl_hook.c	93.18% <0.00%> (-0.08%)	⬇️
src/cache.c	94.93% <0.00%> (-0.07%)	⬇️
... and 13 more

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