LT-22525: Handle missing POS in msa and empty form hvo

[AndyBlack]

This fixes https://jira.sil.org/browse/LT-22525 (Crash occurs when clicking the “Parse this text” button while using the “Use TonePars with FLEx” utility).

There were two problems:

1. A stem MSA with a missing part of speech caused the crash reported in the issue.
2. After fixing that (by using an hvo of 0), processing the result could end up with an empty value for the form.

Both of these are fixed here.

---

This change is 

[github-actions]

NUnit Tests

    1 files  ±0      1 suites  ±0   11m 11s ⏱️ -41s
4 205 tests ±0  4 134 ✅ ±0  71 💤 ±0  0 ❌ ±0 
4 214 runs  ±0  4 143 ✅ ±0  71 💤 ±0  0 ❌ ±0 

Results for commit db1b55f. ± Comparison against base commit dc2572c.