bug: Fix problem with b is None

Author: mrava87

pyproximal/proximal/L2.py

@@ -97,7 +97,7 @@ def __init__(self, Op=None, b=None, q=None, sigma=1., alpha=1.,
         self.count = 0
 
         # create data term
-        if self.Op is not None:
+        if self.Op is not None and self.b is not None:
             self.OpTb = self.sigma * self.Op.H @ self.b
             # create A.T A upfront for explicit operators
             if self.Op.explicit: