EnglishPhonemesTokenizer skips <oov> when encoding
**Describe the bug**

Calling `EnglishPhonemesTokenizer.encode_from_g2p()` with text that a list of phonemes that contains `<oov>` as one of the items simply skips the `<oov>` item even if `<oov>` is a part of the `EnglishPhonemesTokenizer.tokens`.

**Steps/Code to reproduce bug**

```
from nemo.collections.tts.torch.tts_tokenizers import EnglishPhonemesTokenizer
from nemo.collections.tts.torch.g2ps import EnglishG2p

g2p = EnglishG2p(ignore_ambiguous_words=False)
tokenizer = EnglishPhonemesTokenizer(g2p, punct=True, stresses=True)
tokens = tokenizer.encode_from_g2p(['OY1', '<oov>'])
assert len(tokens) == 2
```

**Expected behavior**

Tokenizer to correctly tokenize `<oov>` token