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
diff --git a/bp2build/java_proto_conversion_test.go b/bp2build/java_proto_conversion_test.go
index f546cf4..5d6b088 100644
--- a/bp2build/java_proto_conversion_test.go
+++ b/bp2build/java_proto_conversion_test.go
@@ -68,6 +68,7 @@
         type: "%s",
     },
     srcs: ["a.proto"],
+    sdk_version: "current",
 }`
 
 	protoLibrary := MakeBazelTarget("proto_library", "java-protos_proto", AttrNameToString{
@@ -86,10 +87,12 @@
 					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 @@
     name: "java-protos",
     srcs: ["a.proto"],
     java_version: "7",
+    sdk_version: "current",
 }
 `,
 		ExpectedBazelTargets: []string{
@@ -116,15 +120,20 @@
 				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"`,
+				}),
 		},
 	})
 }