Fix empty unnest columns handling when pushdown_projection_inexact

[zhuqi-lucas]

When we upgrade to DF 49.0.2, there is a critical bug due to unnest changes in DF, this PR fix it in MV.

This PR to add handling to add checking for empty columns when we pushdown partition with unnest column:

        // GUARD: if after pushdown the set of relevant unnest columns is empty,
        // avoid constructing an Unnest node with zero exec columns (which will
        // later error in Unnest::try_new). Instead, simply project the
        // desired output columns from the child plan (after pushing down the child projection).
        // Related PR: https://github.com/apache/datafusion/pull/16632, after that we must
        // also check for empty exec columns here.