Prevent NullPointerException when mainArtifact is not defined

I failed to write unit tests to reproduce the NPE.
This requires more investigation.

PiperOrigin-RevId: 362948108

src/main/java/com/google/devtools/build/lib/rules/python/PyCommon.java

diff --git a/src/main/java/com/google/devtools/build/lib/rules/python/PyCommon.java b/src/main/java/com/google/devtools/build/lib/rules/python/PyCommon.java
index c20e651..d39c0a1 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/python/PyCommon.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/python/PyCommon.java
@@ -277,7 +277,7 @@
           && semantics.prohibitHyphensInPackagePaths()
           && Util.containsHyphen(src.getLabel().getPackageFragment())
           // It's ok to have hyphens in main file - usually no one imports it.
-          && !src.getLabel().equals(mainArtifact.getOwnerLabel())) {
+          && (mainArtifact == null || !src.getLabel().equals(mainArtifact.getOwnerLabel()))) {
         ruleContext.attributeError(
             "srcs",
             src.getLabel()