fix to return empty value for empty header

The method get_http_request_headers_bytes return empty vec for empty header value but get_http_request_header_bytes returns None for empty header value.
Also added same fix in get_map_value for get_http_request_header method.

This commit intends to fix this inconsistency.

Signed-off-by: prembhaskal <pbhaskal@gmail.com>

Author: prembhaskal

src/hostcalls.rs

@@ -241,7 +241,7 @@ pub fn get_map_value(map_type: MapType, key: &str) -> Result<Option<String>, Sta
                         .unwrap(),
                     ))
                 } else {
-                    Ok(None)
+                    Ok(Some(String::new()))
                 }
             }
             Status::NotFound => Ok(None),
@@ -269,7 +269,7 @@ pub fn get_map_value_bytes(map_type: MapType, key: &str) -> Result<Option<Bytes>
                         return_size,
                     )))
                 } else {
-                    Ok(None)
+                    Ok(Some(Vec::new()))
                 }
             }
             Status::NotFound => Ok(None),