Fix Issue #111: NPE when using Android plugin

Added defensive code tp prevent an NPE when this plugin is used
with the Android plugin. I believe this points to a problem in
the Android plugin setting the Test task classpath to a null instead
of an empty file collection as would be the correct way to do it.

README.md

@@ -22,6 +22,17 @@
 
 The plugin provides generation of code coverage reports using [OpenClover](http://openclover.org/).
 
+# Gradle Compatibility Tested
+
+Built for Oracle JDK7
+Tested with Oracle JDK8
+
+| Gradle Version | Works |
+| :------------: | :---: |
+| 2.14.1         | yes   |
+| 3.5.1          | yes   |
+| 4.4.1          | yes   |
+
 ## Usage
 
 To use the Clover plugin, include in your build script:
@@ -39,7 +50,7 @@ example on how to retrieve it from Bintray:
         }
 
         dependencies {
-            classpath 'com.bmuschko:gradle-clover-plugin:2.2.0'
+            classpath 'com.bmuschko:gradle-clover-plugin:2.2.1'
         }
     }
 

RELEASE_NOTES.md

@@ -1,3 +1,7 @@
+### Version 2.2.1 (April 24, 2017)
+
+* Fix NPE with Android Plugin - [Issue 111](https://github.com/bmuschko/gradle-clover-plugin/issues/111)
+
 ### Version 2.2.0 (December 29, 2017)
 
 Gradle 4.x compatibility release

src/functTest/groovy/com/bmuschko/gradle/clover/JavaProjectSpec.groovy

@@ -64,6 +64,6 @@ class JavaProjectSpec extends AbstractFunctionalTestBase {
         cloverPdfReport.exists()
         
         where:
-        gradle << [ '2.14.1', '3.5.1', CURRENT_GRADLE ]
+        gradle << GRADLE_TEST_VERSIONS
     }
 }

src/main/groovy/com/bmuschko/gradle/clover/CloverPlugin.groovy

@@ -114,7 +114,11 @@ class CloverPlugin implements Plugin<Project> {
     @CompileStatic
     private void configureActionsForTask(Test test, Project project, CloverPluginConvention cloverPluginConvention, SourceSetsResolver resolver, AggregateDatabasesTask aggregateDatabasesTask) {
         if (testTaskEnabled(test, cloverPluginConvention)) {
-            test.classpath += project.configurations.getByName(CONFIGURATION_NAME).asFileTree
+            // NB: I believe this is a bug in one of the Android plugins used in the
+            // user's build who reported this in Issue #111, adding some defensive
+            // logic here to avoid adding to a null pointer. In Gradle 4.7 this
+            // might change even further and perhaps will disallow assigning a null.
+            test.classpath = (test.classpath ?: project.files()).plus(project.configurations.getByName(CONFIGURATION_NAME))
             OptimizeTestSetAction optimizeTestSetAction = createOptimizeTestSetAction(cloverPluginConvention, project, resolver, test)
             test.doFirst optimizeTestSetAction // add first, gets executed second
             test.doFirst createInstrumentCodeAction(cloverPluginConvention, project, resolver, test) // add second, gets executed first