fix: prevent segfault in helm.UnmarshalK8SYaml with empty YAML documents

[james00012]

Summary

Fixes a segfault that occurs when helm.UnmarshalK8SYaml processes empty YAML documents (e.g., --- separators without content).

Changes

1. Single object unmarshaling: Added nil check after decoding to return early when encountering empty documents
2. Slice unmarshaling: Added nil check to skip empty documents in the loop
3. Test coverage: Added EmptyDocument test case covering:
   • Single empty document separator (---)
   • Multiple empty separators (---\n---\n---)
   • Empty documents with slice destination

Root Cause

The YAML decoder successfully decodes empty documents but returns nil for rawYaml. The code then attempted to call json.Marshal(rawYaml) on nil, causing a segfault.

Test Plan

• Added unit tests for empty document scenarios
• Verified existing tests still pass
• Confirmed no segfault with empty YAML documents

[jeromepochat]

Looks good to me.
Thanks @james00012 !