Some LSPs like for example neocmakelsp [0] do not mark the diagnostics
with a severity, which would throw an error:
```
Error detected while processing function <SNR>66_Output_cb[5]..lsp#handlers#ProcessMess
ages[32]..lsp#handlers#ProcessNotif[52]..<SNR>67_ProcessDiagNotif[2]..lsp#diag#DiagNoti
fication[72]..lsp#diag#ProcessNewDiags[5]..<SNR>69_SendAleDiags[14]..<lambda>46:
line 6:
E716: Key not present in Dictionary: "severity"
```