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
diff --git a/bp2build/build_conversion.go b/bp2build/build_conversion.go
index 10ee582..eb60cd1 100644
--- a/bp2build/build_conversion.go
+++ b/bp2build/build_conversion.go
@@ -698,9 +698,9 @@
 		} else {
 			return true
 		}
-	// Always print bools, if you want a bool attribute to be able to take the default value, use a
-	// bool pointer instead
-	case reflect.Bool:
+	// Always print bool/strings, if you want a bool/string attribute to be able to take the default value, use a
+	// pointer instead
+	case reflect.Bool, reflect.String:
 		return false
 	default:
 		if !value.IsValid() {
diff --git a/bp2build/build_conversion_test.go b/bp2build/build_conversion_test.go
index 983604b..95a26a9 100644
--- a/bp2build/build_conversion_test.go
+++ b/bp2build/build_conversion_test.go
@@ -41,6 +41,7 @@
     soong_module_deps = [
     ],
     bool_prop = False,
+    string_prop = "",
 )`,
 		},
 		{
@@ -58,6 +59,7 @@
     soong_module_deps = [
     ],
     bool_prop = True,
+    string_prop = "",
 )`,
 		},
 		{
@@ -76,6 +78,7 @@
     ],
     bool_prop = False,
     owner = "a_string_with\"quotes\"_and_\\backslashes\\\\",
+    string_prop = "",
 )`,
 		},
 		{
@@ -94,6 +97,7 @@
     ],
     bool_prop = False,
     required = ["bar"],
+    string_prop = "",
 )`,
 		},
 		{
@@ -111,6 +115,7 @@
     soong_module_deps = [
     ],
     bool_prop = False,
+    string_prop = "",
     target_required = [
         "qux",
         "bazqux",
@@ -147,6 +152,7 @@
         "tag": ".bar",
         "targets": ["goal_bar"],
     }],
+    string_prop = "",
 )`,
 		},
 		{
@@ -179,6 +185,7 @@
     }],
     owner = "custom_owner",
     required = ["bar"],
+    string_prop = "",
     target_required = [
         "qux",
         "bazqux",
@@ -223,11 +230,24 @@
 func TestGenerateBazelTargetModules(t *testing.T) {
 	testCases := []bp2buildTestCase{
 		{
+			description: "string ptr props",
+			blueprint: `custom {
+	name: "foo",
+    string_ptr_prop: "",
+    bazel_module: { bp2build_available: true },
+}`,
+			expectedBazelTargets: []string{
+				makeBazelTarget("custom", "foo", attrNameToString{
+					"string_ptr_prop": `""`,
+				}),
+			},
+		},
+		{
 			description: "string props",
 			blueprint: `custom {
 	name: "foo",
     string_list_prop: ["a", "b"],
-    string_prop: "a",
+    string_ptr_prop: "a",
     bazel_module: { bp2build_available: true },
 }`,
 			expectedBazelTargets: []string{
@@ -236,7 +256,7 @@
         "a",
         "b",
     ]`,
-					"string_prop": `"a"`,
+					"string_ptr_prop": `"a"`,
 				}),
 			},
 		},
@@ -245,7 +265,7 @@
 			blueprint: `custom {
     name: "foo",
     string_list_prop: ["\t", "\n"],
-    string_prop: "a\t\n\r",
+    string_ptr_prop: "a\t\n\r",
     bazel_module: { bp2build_available: true },
 }`,
 			expectedBazelTargets: []string{
@@ -254,7 +274,7 @@
         "\t",
         "\n",
     ]`,
-					"string_prop": `"a\t\n\r"`,
+					"string_ptr_prop": `"a\t\n\r"`,
 				}),
 			},
 		},
diff --git a/bp2build/testing.go b/bp2build/testing.go
index daa9c22..cd84519 100644
--- a/bp2build/testing.go
+++ b/bp2build/testing.go
@@ -149,15 +149,15 @@
 }
 
 type nestedProps struct {
-	Nested_prop string
+	Nested_prop *string
 }
 
 type EmbeddedProps struct {
-	Embedded_prop string
+	Embedded_prop *string
 }
 
 type OtherEmbeddedProps struct {
-	Other_embedded_prop string
+	Other_embedded_prop *string
 }
 
 type customProps struct {
@@ -262,17 +262,17 @@
 }
 
 type EmbeddedAttr struct {
-	Embedded_attr string
+	Embedded_attr *string
 }
 
 type OtherEmbeddedAttr struct {
-	Other_embedded_attr string
+	Other_embedded_attr *string
 }
 
 type customBazelModuleAttributes struct {
 	EmbeddedAttr
 	*OtherEmbeddedAttr
-	String_prop      string
+	String_ptr_prop  *string
 	String_list_prop []string
 	Arch_paths       bazel.LabelListAttribute
 }
@@ -296,7 +296,7 @@
 		paths.ResolveExcludes()
 
 		attrs := &customBazelModuleAttributes{
-			String_prop:      m.props.String_prop,
+			String_ptr_prop:  m.props.String_ptr_prop,
 			String_list_prop: m.props.String_list_prop,
 			Arch_paths:       paths,
 		}