Fix reading error of length property on null

[Digitalone1]

Identify the Bug

Related to #876 (comment)

Description of the Change

Just check if matches is not a falsy js value where the read error occurs.

Alternate Designs

None.

Possible Drawbacks

None since it's good to check if the argument is not falsy when you have to read its properties.

Release Notes

In the same file there's the following comment (which I pasted above the changes):

// The findWordsWithSubsequence method will return `null`
// if the async work was cancelled due to the buffer being
// mutated since it was enqueued.

So the function in then may get null and that causes the issue. I hope this solves the syntax error explained in the link above.