[release/v2.17.x] fix invalid jmx state metrics empty unit (open-telemetry#14195)

Co-authored-by: SylvainJuge <763082+SylvainJuge@users.noreply.github.com>

Author: otelbot[bot]

instrumentation/jmx-metrics/library/src/main/java/io/opentelemetry/instrumentation/jmx/engine/UnitConverter.java

@@ -33,17 +33,12 @@ class UnitConverter {
    * @param sourceUnit a source unit supported by requested converter
    * @param targetUnit a target unit supported by requested converter
    * @return an instance of converter, or {@literal null} if {@code sourceUnit} is {@literal null}
-   *     or empty, which means that there is no conversion needed.
-   * @throws IllegalArgumentException if {@code targetUnit} is empty, or matching converter was not
-   *     found for provided units.
+   *     or empty or if {@code targetUnit} is empty, which means that there is no conversion needed.
+   * @throws IllegalArgumentException if matching converter was not found for provided units.
    */
   @Nullable
   public static UnitConverter getInstance(@Nullable String sourceUnit, String targetUnit) {
-    if (targetUnit.isEmpty()) {
-      throw new IllegalArgumentException("Non empty targetUnit must be provided");
-    }
-
-    if (sourceUnit == null || sourceUnit.isEmpty()) {
+    if (sourceUnit == null || sourceUnit.isEmpty() || targetUnit.isEmpty()) {
       // No conversion is needed
       return null;
     }

instrumentation/jmx-metrics/library/src/test/java/io/opentelemetry/instrumentation/jmx/engine/UnitConverterTest.java

@@ -5,9 +5,9 @@
 
 package io.opentelemetry.instrumentation.jmx.engine;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.junit.jupiter.api.Assertions.assertEquals;
-import static org.junit.jupiter.api.Assertions.assertNull;
 
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
@@ -58,10 +58,11 @@ void shouldHandleUnsupportedConversion(String sourceUnit, String targetUnit) {
   @CsvSource({
     ", s", // null -> "s"
     "'', s", // "" -> "s"
+    "1, ''", // empty target unit
   })
   void shouldSkipConversionWhenSourceUnitNotSpecified(String sourceUnit, String targetUnit) {
     UnitConverter converter = UnitConverter.getInstance(sourceUnit, targetUnit);
-    assertNull(converter);
+    assertThat(converter).isNull();
   }
 
   @Test