Coder is not verified as deterministic in ApproximateUnique.java

Calling input.getCoder() does not verify that the coder is deterministic. A possible example where this could present a problem is if the user's data is float, as the float coder is not deterministic [1].

[1] https://github.com/apache/beam/blob/4d7df122ca053828bd2c7dffa9f390ba2bd8678e/sdks/java/core/src/main/java/org/apache/beam/sdk/coders/FloatCoder.java#L70