Replace falsy tooltip with label

[bwateratmsft]

Needed to fix downstream microsoft/vscode-docker#2679. Alternatively, if microsoft/vscode#115337 is fixed, this will not be needed.

Because all AzExtTreeDataProvider now have resolveTreeItem defined, VSCode always attempts to resolve a tooltip. However, due to this code, if the resolved tooltip is falsy, then it will not replace 'Loading...' from a few lines above.

In the past the standard behavior for tooltips was that label would be used, if tooltip was falsy. So, in this code for resolving tooltip, we'll replace it with the label if it is falsy.

[bwateratmsft]

@nturinski @ejizba @StephenWeatherford Do you know if any extensions other than Docker have started using 0.38.4+ of vscode-azureextensionui? Any using that version will show 'Loading...' as the tooltip forever unless resolveTooltip comes back with something non-falsy, or unless the VSCode team fixes microsoft/vscode#115337. I assume 1.53.0 is due out relatively soon, so if we don't get quick traction from VSCode we should probably put this fix in place and update package users from 0.38.4 to 0.38.5.

[ejizba]

Functions and App Service are on 0.38.4 I think, so we'll make sure to bump them. Thankfully Functions got bumped to 0.38.4 right after we released the extension last week

[StephenWeatherford]

ARM is on an earlier version.

[bwateratmsft]

If we don't hear back on that VSCode bug by say, mid next week, I'll go ahead and put this PR through. What do you think, @nturinski / @ejizba / @StephenWeatherford?

[bwateratmsft]

Per @fiveisprime the upstream fix will probably not be in the nearest upcoming VSCode release, so I'll go ahead and put this fix in.

When it is fixed in VSCode, I'd like to revert the change here, because what we'll be doing is essentially duplicating the logic to use label when tooltip is falsy, but I'd prefer to leave the decision of what to show with VSCode in that scenario. I'll create an issue to revert it later.