mlang/tests: don't call lstrcpyW on a null string

[Dan Kegel]

Avoids the valgrind warning:

Invalid read of size 2
   at lstrcpyW (unicode.h:207)
   by func_mlang (mlang.c:1206)
   by run_test (test.h:556)
   by main (test.h:624)
 Address 0x0 is not stack'd, malloc'd or (recently) free'd
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