TF-IDF: How to handle terms not part of the corpus I'm working on a ML.Net based feature to extract keywords from a document corpus using TD-IDF.

Given this test corpus (one document per line):

This is an example to compute n-grams.
N-gram is a sequence of 'N' consecutive words/tokens.
ML.NET's ProduceNgrams API produces vector of n-grams.
Each position in the vector corresponds to a particular n-gram.
The value at each position corresponds to,
the number of times n-gram occured in the data (Tf), or
the inverse of the number of documents that contain the n-gram (Idf),
or compute both and multiply together (Tf-Idf).


And this test document:

foobar This is an example to compute n-grams


I'm getting the following result:

example             = 2,0794
example|compute     = 2,0794
compute             = 1,3863
compute|n           = 0,0000
n                   = 0,0000
n|grams             = 0,0000
grams               = 0,0000
n|gram              = 0,0000
gram                = 0,0000
gram|sequence       = 0,0000


My question is about the term foobar which is not part of the training corpus, and therefore not part of the result set. Is there a recommended way of dealing with out-of-vocabulary terms when using TF-IDF?
In my opinion, there is no way to deal out-of-vocabulary terms in TF-IDF as it only works using the test corpus. It's like you are trying to find certain word like technology in sports corpus.