Be careful comparing CMake strings to empty, in Findsodium

[zanebeckwith]

Findsodium uses the following to see if the user has specified the name of the sodium libraries (I don't know why it allows that, but it does):
if (sodium_LIBRARIES STREQUAL "")
If this is true, then it sets it to the reasonable default sodium.

The problem is that if sodium_LIBRARIES is not even set, then this is untrue, so sodium_LIBRARIES never gets set, and we call find_library on an unset variable (and thus never find that library).

This fixes that by explicitly augmenting that check to
if (NOT DEFINED sodium_LIBRARIES OR sodium_LIBRARIES STREQUAL "")

[coveralls]

Coverage remained the same at 96.127% when pulling ab97f1a on zanebeckwith:sodium-cmake-str-comp into 8d30167 on xaptum:master.