Fix DirectFunctionCall crash in distributed_exec

[erimatnor]

This change fixes a crash that occurred when calling
distributed_exec via a direct function call.

The crash was triggered by a dynamic lookup of the function name via
the function Oid in the FunctionCallInfo struct in order to generate
error messages for read-only and transaction block checks. However,
this information is provided by the parsing stage, which is not
executed when doing direct function calls, thus leading to a
segmentation fault when trying to dereference a pointer in the
FunctionCallInfo that wasn't set.

Note that this problem is not limited to distributed_exec; it is
present in all SQL-callable functions that use the same pattern and
macros.

To fix the problem, update the macros and patterns used for checking
for read-only mode and transaction blocks to avoid doing the function
name lookup when the pointer is not set. Instead fall back to the C
function name in that case (via C macro __func__).

A test case is added in C code to call distributed_exec via a direct
function call within a transaction block in order to hit the
previously crashing error message.

The distributed_exec function has also been updated with better
handling of input parameters, like empty arrays of data nodes, or
arrays containing NULL elements.

[codecov]

Codecov Report

Merging #3727 (ddebd5b) into master (221437e) will increase coverage by 0.00%.
The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master    #3727   +/-   ##
=======================================
  Coverage   90.29%   90.29%           
=======================================
  Files         212      213    +1     
  Lines       37628    37652   +24     
=======================================
+ Hits        33976    33998   +22     
- Misses       3652     3654    +2     

Impacted Files	Coverage Δ
src/compat/compat.h	94.28% <ø> (ø)
src/process_utility.c	94.34% <ø> (ø)
src/utils.h	80.00% <ø> (ø)
tsl/test/src/test_dist_util.c	85.71% <ø> (ø)
tsl/src/remote/dist_commands.c	92.78% <81.81%> (+0.31%)	⬆️
tsl/test/src/test_dist_commands.c	84.61% <84.61%> (ø)
tsl/src/continuous_aggs/invalidation.c	96.00% <100.00%> (+0.02%)	⬆️
tsl/src/data_node.c	95.47% <100.00%> (ø)

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[pmwkaa]

Looks good

[mkindahl]

LGTM