fix(framework): add null/0 return after while(flag) loops in SolidityNode

Compiler requires explicit return values when while(flag) can exit
without entering the loop body (e.g. during shutdown).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Little-Peony

framework/src/main/java/org/tron/program/SolidityNode.java

@@ -179,6 +179,7 @@ private Block getBlockByNum(long blockNum) {
         sleep(exceptionSleepTime);
       }
     }
+    return null;
   }
 
   private long getLastSolidityBlockNum() {
@@ -195,6 +196,7 @@ private long getLastSolidityBlockNum() {
         sleep(exceptionSleepTime);
       }
     }
+    return 0;
   }
 
   public void sleep(long time) {