fixed dxf_input.py at issue #515

What does the merge request do?

I put a guard code at the beginning of mtext's constructor to skip zero-length phrases.

I also added a unit test and test data for confirmation.

Implementation notes

I no longer get a read error.

Summary for release notes

Users are relieved from the stress of drawing data becoming unconvertible.

Checklist

• Add unit tests (if applicable)
• Changes to inkex/ are well documented
• Clean merge request history