Add artifact only if downloads.artifact is not null, fixes mojang versions that don't list main artifact. (#770)

Author: anatawa12

src/common/java/net/minecraftforge/gradle/common/util/MinecraftRepo.java

@@ -217,7 +217,8 @@ protected File findPom(String side, String version, File json) throws IOExceptio
                 VersionJson meta = Utils.loadJson(json, VersionJson.class);
                 for (VersionJson.Library lib : meta.libraries) {
                     if (lib.isAllowed()) {
-                        builder.dependencies().add(lib.name, "compile");
+                        if (lib.downloads.artifact != null)
+                            builder.dependencies().add(lib.name, "compile");
                         if (lib.downloads.classifiers != null) {
                             if (lib.downloads.classifiers.containsKey("test")) {
                                 builder.dependencies().add(lib.name, "test").withClassifier("test");

src/mcp/java/net/minecraftforge/gradle/mcp/MCPRepo.java

@@ -255,7 +255,8 @@ private File findPom(String side, String version) throws IOException {
                 VersionJson meta = Utils.loadJson(json, VersionJson.class);
                 for (VersionJson.Library lib : meta.libraries) {
                     if (lib.isAllowed()) {
-                        builder.dependencies().add(lib.name, "compile");
+                        if (lib.downloads.artifact != null)
+                            builder.dependencies().add(lib.name, "compile");
                         if (lib.downloads.classifiers != null) {
                             if (lib.downloads.classifiers.containsKey("test")) {
                                 builder.dependencies().add(lib.name, "test").withClassifier("test");