[Improvement] Creating a namespace from just white space should not be allowed

What would you like to be improved?

Update this test to the following, and the test will fail.

  @Test
  public void testFromStringInvalidArgs() {
    Assertions.assertThrows(IllegalArgumentException.class, () -> Namespace.fromString(null));
    Assertions.assertThrows(IllegalArgumentException.class, () -> Namespace.fromString(".a"));
    Assertions.assertThrows(IllegalArgumentException.class, () -> Namespace.fromString("a."));
    Assertions.assertThrows(IllegalArgumentException.class, () -> Namespace.fromString("a..b"));
    Assertions.assertThrows(IllegalArgumentException.class, () -> Namespace.fromString(" "));
  }

How should we improve?

Update Namespace.fromString to throw a IllegalArgumentException if given a namespace that is just whitespace