[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