Fix: Add @nullable annotations to fix Checker Framework nullness errors

Author: xiaojun

linq4j/src/main/java/org/apache/calcite/linq4j/tree/UnsignedType.java

@@ -306,7 +306,7 @@ private static ULong toULongNonNull(Number n, RoundingMode mode) {
     }
     return Unsigned.ulong(s);
   }
-  public static UShort toUShort(Object o) {
+  public static @Nullable UShort toUShort(Object o) {
     if (o == null) {
       return null;
     }
@@ -316,7 +316,7 @@ public static UShort toUShort(Object o) {
     throw new IllegalArgumentException("Cannot cast to UShort: " + o.getClass());
   }
 
-  public static UByte toUByte(Object o) {
+  public static @Nullable UByte toUByte(Object o) {
     if (o == null) {
       return null;
     }
@@ -326,20 +326,14 @@ public static UByte toUByte(Object o) {
     throw new IllegalArgumentException("Cannot cast to UByte: " + o.getClass());
   }
 
-  public static UInteger toUInteger(Object o) {
-    if (o == null) {
-      return null;
-    }
+  public static @Nullable UInteger toUInteger(Object o) {
     if (o instanceof Number) {
       return UInteger.valueOf(((Number) o).longValue());
     }
     throw new IllegalArgumentException("Cannot cast to UInteger: " + o.getClass());
   }
 
-  public static ULong toULong(Object o) {
-    if (o == null) {
-      return null;
-    }
+  public static @Nullable ULong toULong(Object o) {
     if (o instanceof Number) {
       return ULong.valueOf(((Number) o).longValue());
     }