Fix FileDownload with icon

[MarcSkovMadsen]

Closes #6972

Manual Test

import panel as pn

pn.extension()

download = pn.widgets.FileDownload(icon="cloud", file=" ", sizing_mode="stretch_width")
button = pn.widgets.Button(icon="cloud", name="Download", sizing_mode="stretch_width")

download_wo = pn.widgets.FileDownload(file=" ", sizing_mode="stretch_width")
button_wo = pn.widgets.Button(name="Download", sizing_mode="stretch_width")

pn.Column(download, button, download_wo, button_wo).servable()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.72%. Comparing base (7267b38) to head (b5dba7a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6981   +/-   ##
=======================================
  Coverage   81.71%   81.72%           
=======================================
  Files         326      326           
  Lines       48082    48082           
=======================================
+ Hits        39292    39295    +3     
+ Misses       8790     8787    -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.