javascript magic shouldn't execute on nb load

Create nb with a cell:
```
%%javascript
IPython.notebook.kernel.execute('')
```

Now save the notebook, close it and then re-open it. When you do that, under that cell suddenly you will see an error in red:
```
Javascript error adding output!
TypeError: Cannot read property 'execute' of null
See your browser Javascript console for more details.
```
That means the browser has parsed some js code and it tried to run it. This is the error in chrome, it will probably different in a different browser.

Perhaps a more demonstrative example is:
```
%%javascript
alert('Shoud not run')
```
and yet I get an alert pop up as soon as I open the nb in jupyter notebook.

Why do we have the js magic run by the browser?

Moreover this problem happens even if I don't use `%%javascript`, but delegate it to a function:
```
from IPython.display import display, Javascript
def my_code():
display(Javascript("alert('Shoud not run')"))
```
and then have just:
```
my_code()
```
it still pops the alert on nb load.

I found a workaround that prevents the whole js code from running on nb load:

```
%%javascript
if (IPython.notebook.kernel) {
IPython.notebook.kernel.execute('')
}
```
It works since `IPython.notebook.kernel` is not yet defined when the nb just loads.

work environment:
Server 5.7.6
Python 3.7.2
IPython 7.3.0
and then also tested with the latest with the same results.

Thanks.