NullPointerException raised when attempting conversion on null string

I have a type defined as:
```type Some = String Refined MatchesRegex[W.`"[a-zA-Z_][a-zA-Z0-9-_.]*"`.T]```

and then I try to get this type back from a string:
```
//string is a val coming from a call to a java method, which can return null
RefType.applyRef[Some](string)
```

The result is the following exception on runtime:
```
java.lang.NullPointerException:
at eu.timepit.refined.StringValidate.$anonfun$matchesRegexValidate$1(string.scala:129)
at eu.timepit.refined.StringValidate.$anonfun$matchesRegexValidate$1$adapted(string.scala:129)
at eu.timepit.refined.api.Validate$$anon$3.validate(Validate.scala:79)
at eu.timepit.refined.internal.RefinePartiallyApplied.apply(RefinePartiallyApplied.scala:15)
at eu.timepit.refined.internal.ApplyRefPartiallyApplied.apply(ApplyRefPartiallyApplied.scala:19)
...
```

Since applyRef is returning a `Either[String, Some]`, shouldn't that type of error (the string being null) return a `Left(Not allowed empty string)`?