fix: guard None lambda in CLA._solve case-b comparison

Fixes #738. CLA.max_sharpe() raised TypeError: '>' not supported
between instances of 'NoneType' and 'float' whenever all expected
returns were equal.

_compute_lambda returns (None, None) when its denominator c is
exactly zero, which is guaranteed when all expected returns are
identical. The case-a) branch of _solve already guards against this
by comparing CLA._infnone(lam) instead of the raw lam, but the
case-b) branch compared the raw lam directly, causing the crash.

Wraps lam with CLA._infnone(...) in both comparisons of the case-b)
branch, mirroring the existing case-a) pattern. When lam is None this
now resolves to -inf, so the candidate is simply never selected as
i_out/l_out, and the algorithm falls through to the minimum-variance
solution instead of crashing.

Author: saket3395

pypfopt/cla.py

@@ -364,8 +364,8 @@ def _solve(self):
                         self.w[-1][i],
                     )
                     if (
-                        self.ls[-1] is None or lam < self.ls[-1]
-                    ) and lam > CLA._infnone(l_out):
+                        self.ls[-1] is None or CLA._infnone(lam) < self.ls[-1]
+                    ) and CLA._infnone(lam) > CLA._infnone(l_out):
                         l_out, i_out = lam, i
             if (l_in is None or l_in < 0) and (l_out is None or l_out < 0):
                 # 3) compute minimum variance solution