Change proto.encode_text() to omit struct fields with None values.

```bzl
# omit 'bar' field
proto.encode_text(struct(foo=1, bar=None))
```

PiperOrigin-RevId: 511759002
Change-Id: I65034b9da59039429fb55f2a3205f98d47ad38f0

Author: Googler

src/main/java/com/google/devtools/build/lib/packages/StarlarkLibrary.java

@@ -93,6 +93,7 @@ static final class Proto implements StarlarkValue {
                 + "The data structure must be recursively composed of strings, ints, floats, or"
                 + " bools, or structs, sequences, and dicts of these types.\n"
                 + "<p>A struct is converted to a message. Fields are emitted in name order.\n"
+                + "Each struct field whose value is None is ignored.\n"
                 + "<p>A sequence (such as a list or tuple) is converted to a repeated field.\n"
                 + "Its elements must not be sequences or dicts.\n"
                 + "<p>A dict is converted to a repeated field of messages with fields named 'key'"
@@ -108,9 +109,10 @@ static final class Proto implements StarlarkValue {
                 + "# field: 1\n"
                 + "# field: 2\n"
                 + "# field: 3\n\n"
-                + "proto.encode_text(struct(field='text'))\n"
+                + "proto.encode_text(struct(field='text', ignored_field=None))\n"
                 + "# field: \"text\"\n\n"
-                + "proto.encode_text(struct(field=struct(inner_field='text')))\n"
+                + "proto.encode_text(struct(field=struct(inner_field='text',"
+                + " ignored_field=None)))\n"
                 + "# field {\n"
                 + "#   inner_field: \"text\"\n"
                 + "# }\n\n"
@@ -206,6 +208,9 @@ private void field(String name, Object v) throws EvalException {
         }
 
         // non-repeated field
+        if (v == Starlark.NONE) {
+          return;
+        }
         fieldElement(name, v);
       }
 

src/test/java/com/google/devtools/build/lib/starlark/StarlarkRuleClassFunctionsTest.java

@@ -1321,6 +1321,16 @@ public void testSimpleTextMessages() throws Exception {
         "}");
   }
 
+  @Test
+  public void testNoneStructValue() throws Exception {
+    checkTextMessage(
+        "proto.encode_text(struct(a=1, b=None, nested=struct(c=2, d=None)))",
+        "a: 1",
+        "nested {",
+        "  c: 2",
+        "}");
+  }
+
   @Test
   public void testProtoFieldsOrder() throws Exception {
     checkTextMessage("struct(d=4, b=2, c=3, a=1).to_proto()", "a: 1", "b: 2", "c: 3", "d: 4");