Fix MergedNestedJarProvider not calling the child's prepare

src/main/java/net/fabricmc/loom/build/nesting/MergedNestedJarProvider.java

@@ -29,12 +29,19 @@ import java.util.Arrays;
 import java.util.Collection;
 import java.util.stream.Collectors;
 
-public record MergedNestedJarProvider(NestedJarProvider... parents) implements NestedJarProvider {
+import org.gradle.api.Project;
+
+public record MergedNestedJarProvider(NestedJarProvider... children) implements NestedJarProvider {
 	@Override
 	public Collection<File> provide() {
-		return Arrays.stream(parents)
+		return Arrays.stream(children)
 				.map(NestedJarProvider::provide)
 				.flatMap(Collection::stream)
 				.collect(Collectors.toList());
 	}
+
+	@Override
+	public void prepare(Project project) {
+		Arrays.stream(children).forEach(nestedJarProvider -> nestedJarProvider.prepare(project));
+	}
 }

src/main/java/net/fabricmc/loom/build/nesting/NestedJarPathProvider.java

@@ -58,6 +58,8 @@ public final class NestedJarPathProvider implements NestedJarProvider {
 	}
 
 	private void validateFiles() {
+		Preconditions.checkNotNull(files, "null files to nest, was prepare called?");
+
 		for (File file : files) {
 			Preconditions.checkArgument(file.getName().endsWith(".jar"), String.format("Tried to nest %s but it is not a jar", file.getAbsolutePath()));
 			Preconditions.checkArgument(file.exists(), String.format("Tried to nest jar %s but it does not exist", file.getAbsolutePath()));

src/test/groovy/net/fabricmc/loom/test/unit/MergedNestedJarProviderTest.groovy

@@ -0,0 +1,55 @@
+/*
+ * This file is part of fabric-loom, licensed under the MIT License (MIT).
+ *
+ * Copyright (c) 2016, 2017, 2018 FabricMC
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package net.fabricmc.loom.test.unit
+
+import net.fabricmc.loom.build.nesting.MergedNestedJarProvider
+import net.fabricmc.loom.build.nesting.NestedJarProvider
+import org.gradle.api.Project
+import spock.lang.Specification
+
+class MergedNestedJarProviderTest extends Specification {
+    def "empty test"() {
+        given:
+            def nestedJarProvider = new MergedNestedJarProvider(new TestNestedJarProvider())
+        when:
+            nestedJarProvider.prepare(null)
+        then:
+            nestedJarProvider.provide() != null
+    }
+
+    private class TestNestedJarProvider implements NestedJarProvider {
+        private Collection<File> files = null
+
+        @Override
+        Collection<File> provide() {
+            return files
+        }
+
+        @Override
+        void prepare(Project project) {
+            files = []
+        }
+    }
+}