fix c++11 w/ clang

[PPN-SD]

Replace invalid empty‑string initializers with proper boolean values

scim_anthy_prefs.cpp:245:9: error: type 'const char[1]' cannot be narrowed to 'bool' in initializer list [-Wc++11-narrowing]
  245 |         "",
      |         ^~
scim_anthy_prefs.cpp:246:9: error: type 'const char[1]' cannot be narrowed to 'bool' in initializer list [-Wc++11-narrowing]
  246 |         "",
      |         ^~

Sry, I didn't test w/ clang before you released