GH-15809: assign NaN instead of 0 as placeholder value for Loglikelihood

h2o-algos/src/main/java/hex/generic/GenericModel.java

@@ -165,7 +165,7 @@ private Iced getParamByName(String name) {
     public double aic(double likelihood) {
         // calculate negative loglikelihood specifically  for GLM
         if (!_algoName.equals("glm")) {
-            return 0;
+            return Double.NaN;
         } else {
             long betasCount = Arrays.stream(((GlmMojoModelBase) this.genModel()).getBeta()).filter(b -> b != 0).count();
             return -2 * likelihood + 2 * betasCount;
@@ -174,8 +174,10 @@ public double aic(double likelihood) {
 
     @Override
     public double likelihood(double w, double y, double[] f) {
-        // calculate negative loglikelihood specifically  for GLM
-        if (w == 0 || !_algoName.equals("glm")) {
+        // calculate negative loglikelihood specifically for GLM
+        if(!_algoName.equals("glm")) {
+            return Double.NaN;
+        } else if (w == 0) {
             return 0;
         } else {
             // time-consuming calculation for the final scoring for GLM model

h2o-core/src/main/java/hex/Model.java

@@ -1381,11 +1381,11 @@ public double deviance(double w, double y, double f) {
   }
 
   public double likelihood(double w, double y, double[] f) {
-    return 0.0; // place holder.  This function is overridden in GLM.
+    return Double.NaN; // placeholder.  This function is overridden in GLM and GenericModel.
   }
 
   public double aic(double likelihood) {
-    return 0.0; // place holder.  This function is overridden in GLM.
+    return Double.NaN; // placeholder.  This function is overridden in GenericModel.
   }
 
   public ScoringInfo[] scoring_history() { return scoringInfo; }