Additional empty line added to clipboard

[john-bodley]

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• I have checked the superset logs for python stacktraces and included it here as text if any
• I have reproduced the issue with at least the latest released version of superset
• I have checked the issue tracker for the same issue and I haven't found one similar

Superset version

master

Expected results

A regression in the copyToClipboard method which was caused by #3571 causes an undesirable behavior of including a newline in the copied value.

Actual results

This surfaced when someone copying and pasting a slice shortcut,

which gets pasted as (note the empty line):

http://0.0.0.0:8088/r/9

The same is true when copying the query to the clipboard.

to: @mistercrunch @Mogball

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