Empty string cannot be passed in HttpTrigger query string

#### Repro steps

1. Run the following function from templates:
```java
package com.function;

import java.util.*;
import com.microsoft.azure.functions.annotation.*;
import com.microsoft.azure.functions.*;

/**
* Azure Functions with HTTP Trigger.
*/
public class HttpTriggerJava1 {
/**
* This function listens at endpoint "/api/HttpTriggerJava1". Two ways to invoke it using "curl" command in bash:
* 1. curl -d "HTTP Body" {your host}/api/HttpTriggerJava1
* 2. curl {your host}/api/HttpTriggerJava1?name=HTTP%20Query
*/
@FunctionName("HttpTriggerJava1")
public HttpResponseMessage run(
@HttpTrigger(name = "req", methods = {HttpMethod.GET, HttpMethod.POST}, authLevel = AuthorizationLevel.FUNCTION) HttpRequestMessage> request,
final ExecutionContext context) {
context.getLogger().info("Java HTTP trigger processed a request.");

// Parse query parameter
String query = request.getQueryParameters().get("name");
String name = request.getBody().orElse(query);

if (name == null) {
return request.createResponseBuilder(HttpStatus.BAD_REQUEST).body("Please pass a name on the query string or in the request body").build();
} else {
return request.createResponseBuilder(HttpStatus.OK).body("Hello, " + name).build();
}
}
}
```
2. Send the following request: [http://localhost:7071/api/HttpTriggerJava1?name=](http://localhost:7071/api/HttpTriggerJava1?name=)

#### Expected behavior
`name` should be set to an empty string and the function should return "Hello, "

#### Actual behavior
`name` is set to null and the function returns "Please pass a name on the query string or in the request body"

#### Known workarounds
None

#### Related information
Similar issue in PowerShell worker: https://github.com/Azure/azure-functions-powershell-worker/issues/895