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/backend/commands/amcmds.c

@@ -189,6 +189,16 @@ get_index_am_oid(const char *amname, bool missing_ok)
 	return get_am_type_oid(amname, AMTYPE_INDEX, missing_ok);
 }
 
+/*
+ * get_table_am_oid - given an access method name, look up its OID
+ *		and verify it corresponds to an table AM.
+ */
+Oid
+get_table_am_oid(const char *amname, bool missing_ok)
+{
+	return get_am_type_oid(amname, AMTYPE_TABLE, missing_ok);
+}
+
 /*
  * get_am_oid - given an access method name, look up its OID.
  *		The type is not checked.