- This improves the memory usage of `FileStructureElementDiagnosticList`
by avoiding lots of duplicate empty mutable maps. We can assume that
most diagnostics lists are empty, since code is usually green.
- I tested the memory consumption manually by opening all files in
`org.jetbrains.kotlin.idea.k2.codeinsight.inspections` and waiting for
analysis to finish for each file. With the change, the retained size
of `FileStructureElementDiagnosticList` shrank from 63.39kB to
13.77kB.