Fix withKotlin Ant task when no classpath is given

#KT-6492 Fixed

ant/src/org/jetbrains/kotlin/ant/KotlinCompilerAdapter.java

@@ -42,7 +42,12 @@ public class KotlinCompilerAdapter extends DefaultCompilerAdapter {
 
         Kotlin2JvmTask kotlinTask = new Kotlin2JvmTask();
         kotlinTask.setOutput(javac.getDestdir());
-        kotlinTask.setClasspath(javac.getClasspath());
+
+        Path classpath = javac.getClasspath();
+        if (classpath != null) {
+            kotlinTask.setClasspath(classpath);
+        }
+
         kotlinTask.setSrc(javac.getSrcdir());
         kotlinTask.setExternalAnnotations(externalAnnotations);
 

compiler/testData/integration/ant/jvm/noClasspathGiven/J.java

@@ -0,0 +1,4 @@
+package hello;
+
+public class J {
+}

compiler/testData/integration/ant/jvm/noClasspathGiven/build.log.expected

@@ -0,0 +1,14 @@
+OUT:
+Buildfile: [TestData]/build.xml
+
+build:
+    [mkdir] Created dir: [Temp]/classes
+    [javac] Compiling 1 source file to [Temp]/classes
+    [javac] Compiling [[TestData]] => [[Temp]/classes]
+    [javac] Running javac...
+      [jar] Building jar: [Temp]/hello.jar
+
+BUILD SUCCESSFUL
+Total time: [time]
+
+Return code: 0

compiler/testData/integration/ant/jvm/noClasspathGiven/build.xml

@@ -0,0 +1,13 @@
+<project name="Ant Task Test" default="build">
+    <taskdef resource="org/jetbrains/kotlin/ant/antlib.xml" classpath="${kotlin.lib}/kotlin-ant.jar"/>
+
+    <target name="build">
+        <mkdir dir="${temp}/classes"/>
+        <javac srcdir="${test.data}" destdir="${temp}/classes" includeantruntime="false">
+            <withKotlin/>
+        </javac>
+        <jar destfile="${temp}/hello.jar">
+            <fileset dir="${temp}/classes"/>
+        </jar>
+    </target>
+</project>

compiler/testData/integration/ant/jvm/noClasspathGiven/hello.kt

@@ -0,0 +1,5 @@
+package hello
+
+fun main(args: Array<String>) {
+    println("Hello!")
+}

compiler/testData/integration/ant/jvm/noClasspathGiven/hello.run.expected

@@ -0,0 +1,4 @@
+OUT:
+Hello!
+
+Return code: 0

compiler/tests/org/jetbrains/kotlin/integration/AntTaskJvmTest.java

@@ -96,6 +96,11 @@ public class AntTaskJvmTest extends AntTaskBaseTest {
         doJvmAntTest();
     }
 
+    @Test
+    public void noClasspathGiven() throws Exception {
+        doJvmAntTest();
+    }
+
     @Test
     public void javacCompiler() throws Exception {
         doJvmAntTest("-cp", getClassPathForAnt(),