Fix empty Styles pane issue when no style is matched for css variable
Fix empty Styles pane issue when no style is matched for css variable

When there is no matched style for a css variable declaration, the
Styles pane rendering would throw an error, which causes an empty Styles
pane. This CL creates a plain text node instead of throwing in this
case, allowing the rest of the Styles pane to render as expected.

Fixed: chromium:1190406
Change-Id: I4d63b9430a64462dc76a981b06f4ddad5e828233
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2780791
Auto-Submit: Changhao Han <68542faf32f0a558a873c0f094ca9bedb2744bff@chromium.org>
Reviewed-by: Mathias Bynens <17a2cbc4e33156874a9862b0f530521117e66a3d@chromium.org>
Reviewed-by: Alex Rudenko <d8d88e7d7e77f69e23d1d1432231196fb8e02a62@chromium.org>
Reviewed-by: Paul Lewis <8915f5d5c39a08c5a3e8a8d7314756e4580816f9@chromium.org>
Commit-Queue: Changhao Han <68542faf32f0a558a873c0f094ca9bedb2744bff@chromium.org>

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