Change pkg_rpm changelog to optional

The changelog is not used by make_rpm.py, and I don't have a need for it since I auto-generate the SPEC file, which includes a "%changelog" section, prior to invoking the pkg_rpm rule.

Closes #4717.

PiperOrigin-RevId: 187494400
diff --git a/tools/build_defs/pkg/rpm.bzl b/tools/build_defs/pkg/rpm.bzl
index 2db0e82..fe4c6b2 100644
--- a/tools/build_defs/pkg/rpm.bzl
+++ b/tools/build_defs/pkg/rpm.bzl
@@ -45,8 +45,11 @@
   args += ["--out_file=" + ctx.outputs.rpm.path]
 
   # Add data files.
-  files += [ctx.file.changelog] + ctx.files.data
-  args += [ctx.file.changelog.path]
+  if ctx.file.changelog:
+    files += [ctx.file.changelog]
+    args += [ctx.file.changelog.path]
+  files += ctx.files.data
+
   for f in ctx.files.data:
     args += [f.path]
 
@@ -74,7 +77,7 @@
         "architecture": attr.string(default="all"),
         "version_file": attr.label(allow_files=True, single_file=True),
         "version": attr.string(),
-        "changelog" : attr.label(mandatory=True, allow_files=True, single_file=True),
+        "changelog" : attr.label(allow_files=True, single_file=True),
         "data": attr.label_list(mandatory=True, allow_files=True),
 
         # Implicit dependencies.