Continuation of Allow shortcuts to be empty and to have win/mac/linux-specific keys

[groutr]

References

Continues #7335

Code changes

This PR removes jupyterlab checks for empty shortcuts allowing phosphor to handle platform specific keyboard shortcuts.

User-facing changes

Users can define platform specific shortcuts as follows:

{
    "shortcuts": [
        {
            "command": "filebrowser:create-main-launcher",
            "keys": ["Accel Shift L"],
            "selector": "body",
            "disabled": true
        },
        {
            "command": "filebrowser:create-main-launcher",
            "keys": [],
            "linuxKeys": ["Accel Y"],
            "winKeys": ["Accel Shift ;"],
            "selector": "body"
        },
    ]
}

Note that the "keys" list is still required, but can be empty now. As referenced in #7335, this allows phosphor to use "linuxKeys", "winKeys", or "macKeys" depending on the platform.

Backwards-incompatible changes

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