fix a bug in getTables to obtain the amname of heap table.

[li-bupt]

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fix a bug in getTables to obtain the amname of the heap table in lower versions of postgres. In the version upgrade from GPDB6 to GPDB7, amname will be used as the value of default_table_access_method to specify whether the heap table or ao table will be created next. In the getAOTableInfo function, the amname of the ao table is obtained, but the amname of the heap table is missing. This will cause some heap tables to be created as ao tables.

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