[Canvas] Ignore objects with no embedded view

When dropping objects onto canvas to create
panels, ignore any objects that have no
embedded view, in order to avoid nasa/mct#220
[Canvas] Show empty view when none available
[Canvas] Regression test for canvas drop bug

Reproduces nasa/mct#220
[Canvas] Merge regression test into fix branch
[Canvas] Amend drop target test
[Canvas] Remove redundant null check

Return value of getViewInfoForCanvas cannot
be null as of 0805488b8eec19eacafe10fb420bb3f6ed429c08,
so remove check for null introduced as working
solution in 3b70a97130b049c5ae38c9068a09cd34b44ba556
Merge pull request #240 from VWoeltjen/mct220

[Canvas] Avoid exception when no embedded view is available