Fix ValidIdentifierHelper.IsValidIdentifier accepting @ as valid

[SimonCropp]

The else branch stripped the '@' prefix via Substring(1) but never validated the remaining value, so "@" alone was accepted as a valid identifier. Add an empty check after stripping and add tests.

[github-actions]

Test Results

    79 files  + 12      79 suites  +12   1h 38m 14s ⏱️ + 3m 35s
 2 077 tests +  9   2 074 ✅ +  9   3 💤 ±0  0 ❌ ±0 
38 165 runs  +300  38 123 ✅ +300  42 💤 ±0  0 ❌ ±0 

Results for commit 2de87e3. ± Comparison against base commit 40ad85b.