Exclude households with negative earnings from baseline earnings amount for LSRs

[MaxGhenis]

Currently the bottom income decile has negative aggregate earnings, despite us filtering out those with negative net income. This flips the sign of the relative earnings effect compared to the absolute earnings effect. We should exclude those with negative baseline earnings from the LSR in the decile table.

[anth-volk]

See #1741 for a PR accomplishing similar things in non-LSR decile impacts.