Don't list default values for required attributes.

PiperOrigin-RevId: 299138529

src/main/java/com/google/devtools/build/docgen/RuleDocumentationAttribute.java

diff --git a/src/main/java/com/google/devtools/build/docgen/RuleDocumentationAttribute.java b/src/main/java/com/google/devtools/build/docgen/RuleDocumentationAttribute.java
index ad9cf0f..c0e3396 100644
--- a/src/main/java/com/google/devtools/build/docgen/RuleDocumentationAttribute.java
+++ b/src/main/java/com/google/devtools/build/docgen/RuleDocumentationAttribute.java
@@ -202,14 +202,20 @@
     if (attribute == null) {
       return "";
     }
-    StringBuilder sb = new StringBuilder()
-        .append(TYPE_DESC.get(attribute.getType()))
-        .append("; " + (attribute.isMandatory() ? "required" : "optional"))
-        .append(!attribute.isConfigurable()
-            ? String.format("; <a href=\"%s#configurable-attributes\">nonconfigurable</a>",
-                RuleDocumentation.COMMON_DEFINITIONS_PAGE)
-            : "")
-        .append(getDefaultValue());
+    StringBuilder sb =
+        new StringBuilder()
+            .append(TYPE_DESC.get(attribute.getType()))
+            .append("; ")
+            .append(attribute.isMandatory() ? "required" : "optional")
+            .append(
+                !attribute.isConfigurable()
+                    ? String.format(
+                        "; <a href=\"%s#configurable-attributes\">nonconfigurable</a>",
+                        RuleDocumentation.COMMON_DEFINITIONS_PAGE)
+                    : "");
+    if (!attribute.isMandatory()) {
+      sb.append(getDefaultValue());
+    }
     return sb.toString();
   }