Fix TC displaying "TC was not allowed to create folder"

This message was both not needed and wrong.
Let StringHelper.readCommaSeperatedString correctly handle empty lines

Empty lines will now be treated as no elements, instead of one empty
element. Updated the test case to reflect this.

Fixes #268, where TC tried to load an empty biome if no custom biomes
were specified.
Remove some unneeded code, as StringHelper.readCommaSeperatedString already trims values
Don't allow biomes with empty names

The WorldConfig setting CustomBiomes could get corrupted by bug #268.
This commit makes TC skip biomes with empty names, so that empty biomes
written to CustomBiomes will be removed.

From now on, custom biomes with an empty name are disallowed, so
CustomBiomes: Foo:60,:61 also won't work anymore (note the empty biome
with id 61).