Changed default split when working on char_level

[Omerdan03]

Summary

When working i char_level the basic split should be empty string ("") instead of space (" ")

Related Issues

PR Overview

• [n ] This PR requires new unit tests [y/n] (make sure tests are included)
• [y] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
• [y] This PR is backwards compatible [y/n]
• [n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)

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[Omerdan03]

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