Empty/null source entry causes high memory usage

[sophiesaunders]

I've found that when a sources list contains an empty/null entry, XcodeGen experiences extreme memory growth and dramatically slower project generation.

Expected Behavior

XcodeGen rejects the null entry with a validation error, or silently ignores it.

Actual Behavior

XcodeGen appears to interpret the null entry as the project root directory and attempts to include every file as a source. Memory usage in my particular use case grows to ~4.5 GB and generation time increases from ~2 minutes to ~15 minutes.

Environment

XcodeGen version: 2.42.0
macOS version: 15.7.4
Project size: Very large (which likely explains the drastic increase in time and memory consumption)

Reproducing

Here's a simple example, note the empty sources line:

targets:
  MyTarget:
    type: tool
    sources:
      -
      - path/to/actual/source.cpp

This may be expected behavior and not a bug, but it seems highly possible for a user to accidentally introduce a blank source, without intentionally pulling in every source file.