[fix](multi-catalog) Revert #36575 and check nullptr of data column

[AshinGau]

Proposed changes

Revert #36575, because VScanner::get_block will check DCHECK(block->rows() == 0), so block should be cleared when eof = true.

[doris-robot]

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[AshinGau]

run buildall

[github-actions[bot]]

clang-tidy review says "All clean, LGTM! :+1:"

[doris-robot]

TeamCity be ut coverage result: Function Coverage: 36.34% (9151/25185) Line Coverage: 27.88% (74680/267891) Region Coverage: 26.79% (38513/143782) Branch Coverage: 23.48% (19520/83136) Coverage Report: http://coverage.selectdb-in.cc/coverage/8aafc51275fb21c6dcd1adfe2403df7402e76e45_8aafc51275fb21c6dcd1adfe2403df7402e76e45/report/index.html

[github-actions[bot]]

PR approved by at least one committer and no changes requested.

[github-actions[bot]]

PR approved by anyone and no changes requested.