Fix: convert Java enc name and fallback to utf-8

Author: fangerer

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/cext/hpy/GraalHPyContextFunctions.java

@@ -1875,7 +1875,16 @@ static Object doGeneric(GraalHPyContext hpyContext, Object charPtr,
 
         @TruffleBoundary
         static Encoding getFSDefault() {
-            return Encoding.fromJCodingName(System.getProperty("file.encoding"));
+            String fileEncoding = System.getProperty("file.encoding");
+            if (fileEncoding != null) {
+                try {
+                    return Encoding.valueOf(fileEncoding.replace('-', '_'));
+                } catch (IllegalArgumentException e) {
+                    // avoid any fatal Java exceptions; fall through
+                }
+            }
+            // fall back to UTF-8
+            return Encoding.UTF_8;
         }
     }