minor: adding partial tests

Author: Rosander0

src/test/java/com/thealgorithms/machinelearning/MultinomialNaiveBayesClassifierTest.java

@@ -11,7 +11,14 @@ class MultinomialNaiveBayesClassifierTest {
     @Test
     void predictsCorrectClassOnSeparableToyDataset() {
         // Class 0 samples are dominated by feature 0; class 1 samples by feature 1.
-        double[][] features = {{5, 1},{6, 0},{4, 1},{1, 5},{0, 6},{1, 4},};
+        double[][] features = {
+            {5, 1},
+            {6, 0},
+            {4, 1},
+            {1, 5},
+            {0, 6},
+            {1, 4},
+            };
         int[] labels = {0, 0, 0, 1, 1, 1};
 
         MultinomialNaiveBayesClassifier classifier = new MultinomialNaiveBayesClassifier();
@@ -98,4 +105,12 @@ void mismatchedFeatureAndLabelLengthsThrowsIllegalArgumentException() {
         MultinomialNaiveBayesClassifier classifier = new MultinomialNaiveBayesClassifier();
         assertThrows(IllegalArgumentException.class, () -> classifier.fit(features, labels));
     }
+
+    @Test
+    void emptyFeaturesArrayThrowsIllegalArgumentException() {
+        double[][] features = {};
+        int[] labels = {};
+        MultinomialNaiveBayesClassifier classifier = new MultinomialNaiveBayesClassifier();
+        assertThrows(IllegalArgumentException.class, () -> classifier.fit(features, labels));
+    }
 }