Transforming coordinates may throw error when no z is set about proj4js

[MoonE]

Here's a test case for this:

      it('should return null for transform of [0, 0] for EPSG:3413 -> EPSG:3857', function () {
        proj4.defs('EPSG:3413', '+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs');
        var point = proj4.transform(
          proj4.Proj('EPSG:3413'),
          proj4.Proj('EPSG:3857'),
          [0, 0]
        );
        assert.strictEqual(point, null);
      });

[ahocevar]

Thanks for the detailed bug report and test case. I created #451 to fix this.