Fix NPE in StochMathMapping.getProbabilityRate for symbol-less rate expressions

`Expression.getSymbols()` is documented to return null when the expression
has no free symbols (see Expression.java:545). The general-kinetics path
in StochMathMapping was iterating over the result with an unguarded
for-each, producing an NPE whose corpus signature is:

    Cannot read the array length because "<local7>" is null
        at cbit.vcell.mapping.StochMathMapping.getProbabilityRate
            (StochMathMapping.java:145)

This trips when a non-mass-action rate expression flattens to a constant
or otherwise contains no free symbols (e.g. a Michaelis-Menten rate where
all parameters and species have already been resolved upstream). The fix
mirrors the established null-guard pattern already used in
AbstractStochMathMapping.getSubstitutedExpr (line 155).

The general-kinetics fallback in StochasticTransformer is the intended
path for non-mass-action kinetics — it takes the authoritative reaction
rate as a net flux and uses max(rate, 0) as the irreversible forward
rate, which is exactly the right shape for stochastic simulation. The
NPE was preventing that path from completing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jcschaff

vcell-core/src/main/java/cbit/vcell/mapping/StochMathMapping.java

@@ -142,7 +142,11 @@ private Expression getProbabilityRate(ReactionStep reactionStep, GeneralKinetics
 
 		// collect symbolTableEntries for speciesContexts within netRateExpr and replace with concentration parameter
 		netRateExpr = new Expression(netRateExpr);
-		for (String symbol : netRateExpr.getSymbols()) {
+		// Expression.getSymbols() returns null when the expression has no free symbols
+		// (e.g. a constant rate, or a rate that only references already-resolved values).
+		String[] symbols = netRateExpr.getSymbols();
+		for (int i = 0; symbols != null && i < symbols.length; i++) {
+			String symbol = symbols[i];
 			SymbolTableEntry symbolTableEntry = netRateExpr.getSymbolBinding(symbol);
 			if (symbolTableEntry instanceof Kinetics.KineticsProxyParameter proxyParameter
 					&& proxyParameter.getTarget() instanceof SpeciesContext sc) {