misc/androidstudio: check on javaCompile to avoid exception

In commit https://git.io/vQ1gV, the classpath was delayed evaluated to
the task executions. But at Library mode, there will be exception
because the javaCompile involved is not intialized. Add check to fix it.

Change-Id: I23ab2374a704de9fd7bf74ccfaed255045032b3d
Reviewed-on: https://go-review.googlesource.com/48050
Reviewed-by: Elias Naur <elias.naur@gmail.com>
diff --git a/misc/androidstudio/src/main/groovy/org/golang/mobile/GobindPlugin.groovy b/misc/androidstudio/src/main/groovy/org/golang/mobile/GobindPlugin.groovy
index bd6e3e0..6fb7d98 100644
--- a/misc/androidstudio/src/main/groovy/org/golang/mobile/GobindPlugin.groovy
+++ b/misc/androidstudio/src/main/groovy/org/golang/mobile/GobindPlugin.groovy
@@ -216,9 +216,11 @@
 		}
 		def cmd = ["bind", "-i"]
 		// Add the generated R and databinding classes to the classpath.
-		def classpath = project.files(javaCompile.classpath, javaCompile.destinationDir)
-		cmd << "-classpath"
-		cmd << classpath.join(File.pathSeparator)
+		if (javaCompile) {
+			def classpath = project.files(javaCompile.classpath, javaCompile.destinationDir)
+			cmd << "-classpath"
+			cmd << classpath.join(File.pathSeparator)
+		}
 		cmd << "-o"
 		cmd << outputFile.getAbsolutePath()
 		cmd << "-target"