[Canvas] Elements resized to 0 width/height can no longer be selected

[cqliu1]

Kibana version: master c067a77

Describe the bug:
Elements can be resizes until they disappear. If you deselect the element

Steps to reproduce:

1. Add an element
2. Resize the element until it has 0 width or height
3. Click outside of the element to deselect it

Expected behavior:
We should set a minimum width/height of 1 to prevent elements from disappearing

Screenshots (if relevant):

[elasticmachine]

Pinging @elastic/kibana-canvas