Remove useless empty lines from stack traces

This printed things like

```
in function 'f'
        called on standard input

in function 'd'
        called on standard input

in function 'b'
        called on standard input

in function 'a'
        called on standard input

```

As a first step, it removes the empty lines so it's now

```
in function 'f'
        called on standard input
in function 'd'
        called on standard input
in function 'b'
        called on standard input
in function 'a'
        called on standard input
```

See #5434.

tests/invocation/broken-config-continues.err

@@ -4,4 +4,3 @@ syntax-error
 ^
 from sourcing file $XDG_CONFIG_HOME/fish/config.fish
 	called during startup
-