Harden tableam against nonexistant / wrong kind of AMs.

Previously it was allowed to set default_table_access_method to an
empty string. That makes sense for default_tablespace, where that was
copied from, as it signals falling back to the database's default
tablespace. As there is no equivalent for table AMs, forbid that.

Also make sure to throw a usable error when creating a table using an
index AM, by using get_am_type_oid() to implement get_table_am_oid()
instead of a separate copy. Previously we'd error out only later, in
GetTableAmRoutine().

Thirdly remove GetTableAmRoutineByAmId() - it was only used in an
earlier version of 8586bf7ed8.

Add tests for the above (some for index AMs as well).

src/include/access/tableam.h

@@ -1616,7 +1616,6 @@ extern void table_block_parallelscan_startblock_init(Relation rel,
  */
 
 extern const TableAmRoutine *GetTableAmRoutine(Oid amhandler);
-extern const TableAmRoutine *GetTableAmRoutineByAmId(Oid amoid);
 extern const TableAmRoutine *GetHeapamTableAmRoutine(void);
 extern bool check_default_table_access_method(char **newval, void **extra,
 								  GucSource source);