make initial widget size of vega to be auto

[HiroyasuNishiyama]

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Proposed changes

Initial widget size of of ui-vega node is 0x5 as shown in the following screen shot . Since this is an invalid value, this PR makes the initial widget size auto.

Checklist

• I have read the contribution guidelines
• For non-bugfix PRs, I have discussed this change on the forum/slack team.
• I have run grunt to verify the unit tests pass
• I have added suitable unit tests to cover the new/changed functionality