Fix ExplainNode forgetting to set sname for node 'T_TupleSplit'

[fanfuxiaoran]

When node is 'T_TupleSplit' , the sname for it forgotten to set, then the explain command will fail. Just fix it

fix #ISSUE_Number

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[my-ship-it]

Please add a description?

[avamingli]

When node is 'T_TupleSplit' , the sname for it forgotten to set, then the explain command will fail.

Add the cases to test?

[fanfuxiaoran]

When node is 'T_TupleSplit' , the sname for it forgotten to set, then the explain command will fail.

Add the cases to test?

Hard to add cases to cover each type of plannode. Maybe enable the autoexplain extension can help.