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

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

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