Don't put spurious leading semicolon in param doc when Param.allowed_…

…types is empty

When expanding these templates, param.type is empty if the type is not specified for that Param in the StarlarkMethod annotation. For example, that's currently showing up for many of the params on attr methods:
https://docs.bazel.build/versions/master/skylark/lib/attr.html

RELNOTES: None.
PiperOrigin-RevId: 344043655

src/main/java/com/google/devtools/build/docgen/templates/skylark-library.vm

@@ -57,10 +57,13 @@ ${module.documentation}
               <code>${param.name}</code>
             </td>
             <td>
+            #if (!$param.type.isEmpty())
+              <code>${param.type}</code>;
+            #end
             #if (!$param.defaultValue.isEmpty())
-             <code>${param.type}</code>; default = ${param.defaultValue}<br/>
+             default = ${param.defaultValue}<br/>
             #else
-             <code>${param.type}</code>; required<br/>
+             required<br/>
             #end
               ${param.documentation}
             </td>