sh_binary: Use pointers for optional attributes

Empty strings for filename and sub_dir were unnecessicarily always being
propagated to BUILD files.

Test: build/bazel/ci/bp2build.sh
Change-Id: I39d6af12b1decd1d3813b92daa862ebc025daa7d

bp2build/sh_conversion_test.go

@@ -73,3 +73,20 @@ func TestShBinarySimple(t *testing.T) {
 			})},
 	})
 }
+
+func TestShBinaryDefaults(t *testing.T) {
+	runShBinaryTestCase(t, bp2buildTestCase{
+		description:                "sh_binary test",
+		moduleTypeUnderTest:        "sh_binary",
+		moduleTypeUnderTestFactory: sh.ShBinaryFactory,
+		blueprint: `sh_binary {
+    name: "foo",
+    src: "foo.sh",
+    bazel_module: { bp2build_available: true },
+}`,
+		expectedBazelTargets: []string{
+			makeBazelTarget("sh_binary", "foo", attrNameToString{
+				"srcs": `["foo.sh"]`,
+			})},
+	})
+}