Fix assertion failure in pg_prewarm() on objects without storage.

An assertion test added in commit 049ef33 could fail when pg_prewarm()
was called on objects without storage, such as partitioned tables.
This resulted in the following failure in assert-enabled builds:

    Failed Assert("RelFileNumberIsValid(rlocator.relNumber)")

Note that, in non-assert builds, pg_prewarm() just failed with an error
in that case, so there was no ill effect in practice.

This commit fixes the issue by having pg_prewarm() raise an error early
if the specified object has no storage. This approach is similar to
the fix in commit 4623d7144 for pg_freespacemap.

Back-patched to v17, where the issue was introduced.

Author: Masahiro Ikeda <ikedamsh@oss.nttdata.com>
Reviewed-by: Dilip Kumar <dilipbalaut@gmail.com>
Reviewed-by: Richard Guo <guofenglinux@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/e082e6027610fd0a4091ae6d033aa117@oss.nttdata.com
Backpatch-through: 17

contrib/pg_prewarm/meson.build

@@ -29,6 +29,11 @@ tests += {
   'name': 'pg_prewarm',
   'sd': meson.current_source_dir(),
   'bd': meson.current_build_dir(),
+  'regress': {
+    'sql': [
+      'pg_prewarm',
+    ],
+  },
   'tap': {
     'tests': [
       't/001_basic.pl',