ignore undefined in onDidChangeActiveTextEditor
ignore undefined in onDidChangeActiveTextEditor

Ignore undefined elements that onDidChangeActiveTextEditor can (and is
documented to) send to licenser. This cleans up a lot of noise in
vscode's `Log (Window)` but doesn't change licenser's behavior since
licenser would crash on the `e.document.fileName` later in the code.

ymotongpoo/vsc-licenser,ymotongpoo/vsc-licenser