while: condition error not diagnosed

The condition

set(paren "(")
while(${paren})
endwhile()

is incorrectly accepted by while.

The same condition:

set(paren "(")
if(${paren})
endif()

is rejected by if:

CMake Error at unbalanced-if.cmake:2 (if):
  if given arguments:

    "("

  mismatched parenthesis in condition

We need to add a policy to reject it in while too.