fix: handle Null Dereference in UnitsConverter

Author: vil02

.inferconfig

@@ -2,7 +2,6 @@
     "report-block-list-path-regex": [
         "src/main/java/com/thealgorithms/ciphers/a5/CompositeLFSR.java",
         "src/main/java/com/thealgorithms/conversions/RomanToInteger.java",
-        "src/main/java/com/thealgorithms/conversions/UnitsConverter.java",
         "src/main/java/com/thealgorithms/datastructures/crdt/GCounter.java",
         "src/main/java/com/thealgorithms/datastructures/crdt/PNCounter.java",
         "src/main/java/com/thealgorithms/datastructures/graphs/KahnsAlgorithm.java",

src/main/java/com/thealgorithms/conversions/UnitsConverter.java

@@ -3,6 +3,7 @@
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
+import java.util.NoSuchElementException;
 import java.util.Set;
 import org.apache.commons.lang3.tuple.Pair;
 
@@ -77,7 +78,7 @@ public double convert(final String inputUnit, final String outputUnit, final dou
             throw new IllegalArgumentException("inputUnit must be different from outputUnit.");
         }
         final var conversionKey = Pair.of(inputUnit, outputUnit);
-        return conversions.get(conversionKey).convert(value);
+        return conversions.computeIfAbsent(conversionKey, k -> { throw new NoSuchElementException("No converter for: " + k); }).convert(value);
     }
 
     public Set<String> availableUnits() {