Remove literal null evidence from assembly loading

Select the existing non-Evidence Assembly.Load or LoadFrom overload when the obsolete two-argument BCL call passes literal null evidence. Retain the load operation, argument evaluation and resolved target overload. Non-null/computed evidence, other overloads, unresolved calls, lambda/query contexts, discarded comments and conditional snapshots remain unchanged. This API preparation does not choose an AssemblyLoadContext or emulate Framework security policy. LoadFile retains an unconditional Framework ControlEvidence demand and is left for review.