Support empty strings in bp2build

Previously, could not set an empty string as a value of an attribute;
however, this is necessary in some cases. To not unnecessarily create an
empty string, use string pointers for attributes rather than strings.

Test: go test bp2build tests
Change-Id: I03b3a3567452d455246d22d81f86c317d06b7c39

python/binary.go

@@ -35,10 +35,10 @@ func registerPythonBinaryComponents(ctx android.RegistrationContext) {
 }
 
 type bazelPythonBinaryAttributes struct {
-	Main           string
+	Main           *string
 	Srcs           bazel.LabelListAttribute
 	Deps           bazel.LabelListAttribute
-	Python_version string
+	Python_version *string
 }
 
 func PythonBinaryBp2Build(ctx android.TopDownMutatorContext) {
@@ -52,12 +52,12 @@ func PythonBinaryBp2Build(ctx android.TopDownMutatorContext) {
 		return
 	}
 
-	var main string
+	var main *string
 	for _, propIntf := range m.GetProperties() {
 		if props, ok := propIntf.(*BinaryProperties); ok {
 			// main is optional.
 			if props.Main != nil {
-				main = *props.Main
+				main = props.Main
 				break
 			}
 		}
@@ -69,13 +69,13 @@ func PythonBinaryBp2Build(ctx android.TopDownMutatorContext) {
 	// under Bionic.
 	py3Enabled := proptools.BoolDefault(m.properties.Version.Py3.Enabled, false)
 	py2Enabled := proptools.BoolDefault(m.properties.Version.Py2.Enabled, false)
-	var python_version string
+	var python_version *string
 	if py3Enabled && py2Enabled {
 		panic(fmt.Errorf(
 			"error for '%s' module: bp2build's python_binary_host converter does not support "+
 				"converting a module that is enabled for both Python 2 and 3 at the same time.", m.Name()))
 	} else if py2Enabled {
-		python_version = "PY2"
+		python_version = &pyVersion2
 	} else {
 		// do nothing, since python_version defaults to PY3.
 	}