SCC: is_code() can return true even if the word is not a code

[palemieux]

From @atsampson:

ttconv/src/main/python/ttconv/scc/word.py

Line 115 in 536f1c8

def is_code(self) -> bool:

is_code() can return True even if self.code is None, i.e., no code was found in the constructor.

See #427