UTF-16 string terminator

7.24.4.6.1 The wcslen function (from the Standard)

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   [#3]   The  wcslen  function  returns  the  number  of  wide
   characters that precede the terminating null wide character.

And the null wide character is L'\0'

Unicode does not define string terminators. Your environment or language does. For instance, C strings use 0x0 as a string terminator, as well as in .NET strings where a separate value in the String class is used to store the length of the string.

To answer your second question, wcslen looks for a terminating L'\0' character. Which as I read it, is any length of 0x00 bytes, depending on the compiler, but will likely be the two-byte sequence 0x00 0x00 if you're using UTF-16 (encoding U+0000, 'NUL')