[lldb/crashlog] Skip null image dsym fetching on interactive mode

Sometimes, it can happen that a crash report has null images in its list
of used binaries. This manifests like such:

```
    0x0 - 0xffffffffffffffff ??? (*) <00000000-0000-0000-0000-000000000000> ???
```

When fetching debug symbols to symbolicate the crashlog stackframe,
having null images causes `dsymForUUID` to hang for few seconds.

This patch addresses that by skipping null images from being load by the
scripted process.

rdar://97419487

Differential Revision: https://reviews.llvm.org/D131038



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>