Fix FunctionOperator copy method to handle nothing values

The FunctionOperator copy method was attempting to copy L.u and deepcopy L.p
even when they were nothing, which causes errors. This fixes the copy method
to check if the values are nothing before attempting to copy them.

Changes:
- Check L.u !== nothing before calling copy(L.u)
- Check L.p !== nothing before calling deepcopy(L.p)

This prevents MethodError when copying FunctionOperators with nothing
values for u or p parameters.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

src/func.jl

@@ -447,8 +447,8 @@ function Base.copy(L::FunctionOperator)
         L.op_inverse,
         L.op_adjoint_inverse,
         L.traits,
-        isdefined(L, :u) ? copy(L.u) : nothing,
-        isdefined(L, :p) ? deepcopy(L.p) : nothing,
+        isdefined(L, :u) && L.u !== nothing ? copy(L.u) : nothing,
+        isdefined(L, :p) && L.p !== nothing ? deepcopy(L.p) : nothing,
         L.t,
         L.cache === nothing ? nothing : deepcopy(L.cache),
         typeof(L).parameters[end-1],  # iType