fix init cell extension bug on trust warning

[lspvic]

When load a Jupyter notebook, the trust warning model dialog always displayed and init cell are not executed although I am very sure that the notebook is trusted.
I debugged the script, found that the notebook's trust property was not initiated with null when checking, as below:

this is because the init_cell extension is loaded before notebook is fully loaded. I found when the notebook is big or the network is slow, the bug occurs, so I do a check on whether the notebook is loaded, if not, binding an notebook_loaded event to execute the init run.

[jcb91]

Good spot, neat fix, thanks @lspvic!