t/gitweb-lib: Don't pass constant to decode_utf8

t/gitweb-lib: Don't pass constant to decode_utf8

Encode.pm started updating the string to decode in-place when a second
argument is passed in version 2.40.

This causes 'decode_utf8("", Encode::FB_CROAK)' to die with a message
like:

  Modification of a read-only value attempted at .../Encode.pm line 216.

Work around this by passing an empty variable instead of a constant
string.

Signed-off-by: Brian Gernhardt <brian@gernhardtsoftware.com>
Acked-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>