trunk/98a0190638f271ea6e37e7e5891ea9823d11baae: Handle empty linalg.lstsq inputs in shared code (#199696)

What happened

linalg.lstsq handed degenerate shapes straight to the backend, so each one inherited LAPACK's quick-return behavior, and those disagree with each other and with the math: gels zeroes B when min(m, n, nrhs) == 0 , so for n == 0 CPU reported zero residuals instead of ||B||^2 (and the backward, which assumes the true residual, disagreed with them). gelsy sets rank = 0 when nrhs == 0 , although the rank of A doesn't dep…

Summary assembled by rule from the sources below

Sources