[ML] Route resolver fails to load _fields_for_wildcard

[walterra]

Due to a change in how Kibana index patterns are initialized, loading certain pages that call the page's useResolver() with '' instead of undefined may fail to load because a call to _fields_for_wildcard fails to return for large clusters.

Investigation showed that until v7.9 in such a case a call to _fields_for_wildcard would not be triggered. The issue appears to happen on v7.10.0+.

The problem is that passing an empty string will end up for _fields_for_wildcard being called with ?pattern= which will try to return all field information for all indices.

The following pages are affected, they use useResolver('', ...):

• Single Metric Viewer (/timeseriesexplorer)
• Analytics Exploration (/data_frame_analytics/exploration)
• Analytics Job List (/data_frame_analytics)
• Analytics Models List (/data_frame_analytics/models)
• File Data Visualizer (/filedatavisualizer)