java-printf-format: Reject null precision.

java-printf-format: Reject null precision.

Proof that it's invalid:
--------------------------- foo.java ---------------------------
public class foo {
  public static void main(String[] args) {
    System.out.println(String.format("%.f\n", 3.1415926535));
  }
}
----------------------------------------------------------------

* gettext-tools/tests/format-java-printf-1: Add a test case with null precision.