Fix LT-21867: Optional contexts don't work properly in Hermit Crab

src/SIL.Machine.Morphology.HermitCrab/PhonologicalRules/RewriteRuleSpec.cs

@@ -85,7 +85,7 @@ out PhonologicalSubruleMatch subruleMatch
                     leftNode,
                     varBindings
                 );
-                if (leftEnvMatch == null || leftEnvMatch.Success)
+                if (leftEnvMatch == null || leftEnvMatch.Success || subruleSpec.LeftEnvironmentMatcher.AcceptsEmpty())
                 {
                     if (leftEnvMatch != null && leftEnvMatch.VariableBindings != null)
                         varBindings = leftEnvMatch.VariableBindings;
@@ -95,7 +95,11 @@ out PhonologicalSubruleMatch subruleMatch
                         rightNode,
                         varBindings
                     );
-                    if (rightEnvMatch == null || rightEnvMatch.Success)
+                    if (
+                        rightEnvMatch == null
+                        || rightEnvMatch.Success
+                        || subruleSpec.RightEnvironmentMatcher.AcceptsEmpty()
+                    )
                     {
                         if (rightEnvMatch != null && rightEnvMatch.VariableBindings != null)
                             varBindings = rightEnvMatch.VariableBindings;

src/SIL.Machine/FiniteState/Fst.cs

@@ -2130,5 +2130,19 @@ public int GetFrozenHashCode()
         {
             return GetHashCode();
         }
+
+        internal bool IsAcceptingState(State<TData, TOffset> state)
+        {
+            State<TData, TOffset> curState = state;
+            while (!curState.IsAccepting)
+            {
+                Arc<TData, TOffset> highestPriArc = curState.Arcs.MinBy(a => a.Priority);
+                if (!highestPriArc.Input.IsEpsilon)
+                    break;
+                curState = highestPriArc.Target;
+            }
+
+            return curState.IsAccepting;
+        }
     }
 }

src/SIL.Machine/Matching/Matcher.cs

@@ -336,5 +336,10 @@ private Annotation<TOffset> GetStartAnnotation(TData input, TOffset start)
                 startAnn = startAnn.GetNextDepthFirst(_settings.Direction, _settings.Filter);
             return startAnn;
         }
+
+        public bool AcceptsEmpty()
+        {
+            return _fsa.IsAcceptingState(_fsa.StartState);
+        }
     }
 }

tests/SIL.Machine.Morphology.HermitCrab.Tests/PhonologicalRules/RewriteRuleTests.cs

@@ -47,6 +47,12 @@ public void SimpleRules()
             new RewriteSubrule
             {
                 Rhs = Pattern<Word, ShapeNode>.New().Annotation(asp).Value,
+                // the following should be a NOOP because it accepts the empty string.
+                LeftEnvironment = Pattern<Word, ShapeNode>
+                    .New()
+                    .Annotation(nonCons)
+                    .Optional.Annotation(nonCons)
+                    .Optional.Value,
                 RightEnvironment = Pattern<Word, ShapeNode>.New().Annotation(nonCons).Value
             }
         );