GP-6452 fix NPE in rust demangler that caused log message spam

Either way the symbol doesn't get demangled, but doesn't need to spam
log with error messages.

Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/analysis/rust/demangler/RustDemangler.java

@@ -61,6 +61,10 @@ public class RustDemangler implements Demangler {
 			demangled = RustDemanglerV0.demangle(mangled);
 		}
 
+		if (demangled == null) {
+			return null;
+		}
+
 		RustDemanglerParser parser = new RustDemanglerParser();
 		DemangledObject demangledObject = parser.parse(mangled, demangled);