Don't assume footer is present in _ConsoleOutputState builder.

[biggs0125]

Resolves this errors:

Null check operator used on a null value
file:///b/s/w/ir/x/w/devtools/packages/devtools_app/lib/src/shared/console/console.dart 214:37 \| build
file:///b/s/w/ir/x/w/rc/flutter/packages/flutter/lib/src/widgets/framework.dart 5824:27 \| build
file:///b/s/w/ir/x/w/rc/flutter/packages/flutter/lib/src/widgets/framework.dart 5875:11 \| performRebuild
file:///b/s/w/ir/x/w/rc/flutter/packages/flutter/lib/src/widgets/framework.dart 5866:11 \| _firstBuild
file:///b/s/w/ir/x/w/rc/flutter/packages/flutter/lib/src/widgets/framework.dart 5692:5 \| mount

[elliette]

Thanks! One request but otherwise LGTM.

I think the test failures might be flakes, I'm re-running them to see.