Add do not convert for sdk_version unset

When sdk_version is unset, the build with fail with Bazel because
private APIs are do not build with Bazel at this time.

This instead prevents migrating the relevant target to Bazel in bp2build

Test: go tests
Change-Id: I31dabcf143de7933706d1b734ef21467a01b65e9

bp2build/java_proto_conversion_test.go

@@ -68,6 +68,7 @@ func TestJavaProto(t *testing.T) {
         type: "%s",
     },
     srcs: ["a.proto"],
+    sdk_version: "current",
 }`
 
 	protoLibrary := MakeBazelTarget("proto_library", "java-protos_proto", AttrNameToString{
@@ -86,10 +87,12 @@ func TestJavaProto(t *testing.T) {
 					tc.javaLibraryType,
 					javaLibraryName,
 					AttrNameToString{
-						"deps": `[":java-protos_proto"]`,
+						"deps":        `[":java-protos_proto"]`,
+						"sdk_version": `"current"`,
 					}),
 				MakeBazelTarget("java_library", "java-protos", AttrNameToString{
-					"exports": fmt.Sprintf(`[":%s"]`, javaLibraryName),
+					"exports":     fmt.Sprintf(`[":%s"]`, javaLibraryName),
+					"sdk_version": `"current"`,
 				}),
 				MakeNeverlinkDuplicateTarget("java_library", "java-protos"),
 			},
@@ -104,6 +107,7 @@ func TestJavaProtoDefault(t *testing.T) {
     name: "java-protos",
     srcs: ["a.proto"],
     java_version: "7",
+    sdk_version: "current",
 }
 `,
 		ExpectedBazelTargets: []string{
@@ -116,15 +120,20 @@ func TestJavaProtoDefault(t *testing.T) {
 				AttrNameToString{
 					"deps":         `[":java-protos_proto"]`,
 					"java_version": `"7"`,
+					"sdk_version":  `"current"`,
 				}),
 			MakeBazelTarget("java_library", "java-protos", AttrNameToString{
 				"exports":      `[":java-protos_java_proto_lite"]`,
 				"java_version": `"7"`,
+				"sdk_version":  `"current"`,
 			}),
 			MakeNeverlinkDuplicateTargetWithAttrs(
 				"java_library",
 				"java-protos",
-				AttrNameToString{"java_version": `"7"`}),
+				AttrNameToString{
+					"java_version": `"7"`,
+					"sdk_version":  `"current"`,
+				}),
 		},
 	})
 }