[dbmanager] Fixes empty table

[pblottiere]

Description

This PR fixes an issue introduced by 777fdeb.

See https://issues.qgis.org/issues/18352.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
• Commits which change the UI or existing user workflows are tagged with [needs-docs] in the commit message and containt sufficient information in the commit message to be documented
• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit

[Gustry]

LGTM, it works. Thanks for the fix!

[pblottiere]

I launched Travis builds several times without success due to an issue with ProcessingQgisAlgorithmsTest.

Considering that it has nothing to do with the current bugfix, I'll merge this PR tomorrow if i have no objections in the meantime.

[nyalldawson]

That should be fixed now - I think some library on travis updated and threw out the reference hash value.

[pblottiere]

Nice, thank you @nyalldawson, Travis is green now :).