fix(api): handle missing attribute in is_multi_lingual

[eginhard]

Copied from coqui-ai#3450:

Fixes coqui-ai#3449, i.e. the following works again:

from TTS.api import TTS

cloud = TTS(model_name="tts_models/de/thorsten/vits")  # just to download the model
_ = cloud.tts("test")  # this works fine

from TTS.utils.generic_utils import get_user_data_dir
model = os.path.join(get_user_data_dir("tts"), "tts_models--de--thorsten--vits", "model_file.pth")
config = os.path.join(get_user_data_dir("tts"), "tts_models--de--thorsten--vits", "config.json")
local = TTS(model_path=model, config_path=config)
_ = local.tts("test")  # this failed previously

Also fixes coqui-ai#3602