Check for null executable when building proto compile lang plugin actions.

PiperOrigin-RevId: 342053686

src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java

diff --git a/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java b/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
index 1ee6d97..9931239 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCompileActionBuilder.java
@@ -245,7 +245,7 @@
   CustomCommandLine.Builder createProtoCompilerCommandLine() {
     CustomCommandLine.Builder result = CustomCommandLine.builder();
 
-    if (langPlugin != null) {
+    if (langPlugin != null && langPlugin.getExecutable() != null) {
       // We pass a separate langPlugin as there are plugins that cannot be overridden
       // and thus we have to deal with "$xx_plugin" and "xx_plugin".
       result.addFormatted(