acr purge command with missing ago parameter can cause --dry-run parameter not to be parsed

**Describe the bug**
missing value for --ago parameter can cause unintended consequences if --dry-run is the last parameter used. Looks like the system ignores the --dry-run parameter and executes the command. It would be nice to have a parse error instead

**To Reproduce**
Steps to reproduce the behavior:
1. PURGE_CMD="acr purge --filter 'anhamilt/dotnet/core/sdk:latest' --ago --dry-run"
2. az acr run --cmd "$PURGE_CMD" --registry myregistrytocleanup /dev/null

**Expected behavior**
Parsing error stating no value was provided for --ago

**Screenshots**