Fix two issues in LambdifyCSE
Fix two issues in LambdifyCSE

1. sympy doesn't sympify its inputs, so sympify before sending to sympy
2. subs can be empty and the code doesn't handle that

symengine/symengine.py,symengine/symengine.py,bjodah/symengine.py,bjodah/symengine.py,symengine/symengine.py,bjodah/symengine.py