estimateMemorySize() throws NPE in MultiPathImpl when the geometry is empty

[tdcmeehan]

Each of these will throw a NPE:

        OGCGeometry.fromText("LINESTRING EMPTY").estimateMemorySize();
        OGCGeometry.fromText("MULTILINESTRING EMPTY").estimateMemorySize();
        OGCGeometry.fromText("GEOMETRY EMPTY").estimateMemorySize();
        OGCGeometry.fromText("MULTIGEOMETRY EMPTY").estimateMemorySize();

Expected: it should not matter if it's empty, it should return the estimated memory size without error.

[tdcmeehan]

I opened #193 for this.