Gradle, js, tests: don't add prefix if platform and/or sub target name is null

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/subtargets/KotlinJsSubTarget.kt

@@ -79,7 +79,9 @@ abstract class KotlinJsSubTarget(
             }
 
             testJs.compilation = compilation
-            testJs.targetName = target.disambiguationClassifier + "," + disambiguationClassifier
+            testJs.targetName = listOfNotNull(target.disambiguationClassifier, disambiguationClassifier)
+                .takeIf { it.isNotEmpty() }
+                ?.joinToString()
 
             testJs.configureConventions()
         }

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/targets/js/testing/karma/KotlinKarma.kt

@@ -270,7 +270,9 @@ class KotlinKarma(override val compilation: KotlinJsCompilation) : KotlinJsTestF
                         val browser = withoutOs.substring(rawSuiteNameOnly.length + 1) // HeadlessChrome 74.0.3729
                             .replace(" ", "") // HeadlessChrome74.0.3729
 
-                        testNameSuffix = "$baseTestNameSuffix,$browser,$os"
+                        testNameSuffix = listOfNotNull(baseTestNameSuffix, browser, os)
+                            .takeIf { it.isNotEmpty() }
+                            ?.joinToString()
 
                         return rawSuiteNameOnly.replace(" ", ".") // sample.a.DeepPackageTest.Inner
                     }