Allow DB without table prefix

[ctueck]

If I see and notice correctly, the current entrypoint.sh scripts do not allow to unset DB_TABLE_PREFIX. When it is set to an empty string it always falls back to the default lime_.

(I am planning to migrate an existing Limesurvey installation to Docker, and our install has no prefix.)

[martialblog]

Hey, you can also provide an existing config.php then the entrypoint will skip all the initialization stuff.

But yeah, makes sense to allow an empty prefix.

Cheers
Markus

[martialblog]

@ctueck Hi, I just pushed some code that should fix this. See PR, feel free to test it.