title: manifest issue for shadow plugin with gradle
body: <p>I am using gradle v3.4 &amp; shadow plugin v1.2.4.  I am publishing a jar file to my local maven repo using the following inside my build.gradle file</p>
  
  <pre><code>mainClassName = 'some.thing.SomeClient'
      jar {
          manifest {
              attributes(
                      'Class-Path': configurations.compile.collect { it.getName() }.join(' '),
                      // 'Main-Class': 'some.thing.SomeClient'
              )
          }
      }
  
      shadowJar {
          baseName = 'commons-java'
          classifier = null
          version = '0.0.1-SNAPSHOT'
      }
  
      artifacts {
          archives shadowJar
      }
  
      jar.dependsOn shadowJar
  </code></pre>
  
  <p>After publishing, I try to use this dependency inside another project as follows but get the error copied below when I run <strong>gradle build</strong></p>
  
  <pre><code> /**
   * jar/shadow jar (shadow jar extends jar task to create fat jar)
   */
  jar {
      manifest {
          attributes(
                  'Class-Path': configurations.compile.collect { it.getName() }.join(' '),
                  //'Main-Class': 'some.thing.SomeClient'
          )
      }
  }
  
  shadowJar {
      baseName = 'something-java-client'
      classifier = null
      version = '0.0.1-SNAPSHOT'
  }
  
  artifacts {
      archives shadowJar
  }
  
  jar.dependsOn shadowJar
  </code></pre>
  
  <p><strong>error</strong></p>
  
  <pre><code>The value of a manifest attribute must not be null (Key=Main-Class).
  </code></pre>