[aws-xray-recorder-sdk-apache-http] Don't return a null target to avoid possible NPE

[srprash]

Issue:
The Apache HTTP instrumentation is prone to throwing a NullPointerException by returning a null from the determineTarget method. Since a user is not expected to handle NPE, it will break the application.

Description of changes:
The method doesn't need to check for URL absoluteness and bail out early if it is not. The URIUtils.extractHost(requestUri) method does this check implicitly and we handle it's null return value by throwing a ClientProtocolException which a user will handle in their code.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[willarmiros]

Looks good to me! Just want to confirm: The test case threw a NPE before the fix, and now just throws a ClientProtocolException? If so, then good to merge

[srprash]

Just want to confirm: The test case threw a NPE before the fix, and now just throws a ClientProtocolException?

Yes, it did. Running the same unit test without the fix:

TracedHttpClientTest > testExceptionOnRelativeUrl FAILED
    java.lang.AssertionError: unexpected exception type thrown; expected:<org.apache.http.client.ClientProtocolException> but was:<java.lang.NullPointerException>
        at org.junit.Assert.assertThrows(Assert.java:1020)
        at org.junit.Assert.assertThrows(Assert.java:981)
        at com.amazonaws.xray.proxies.apache.http.TracedHttpClientTest.testExceptionOnRelativeUrl(TracedHttpClientTest.java:124)
        Caused by:
        java.lang.NullPointerException
            at com.amazonaws.xray.proxies.apache.http.TracedHttpClient.execute(TracedHttpClient.java:180)
            at com.amazonaws.xray.proxies.apache.http.TracedHttpClientTest.lambda$testExceptionOnRelativeUrl$0(TracedHttpClientTest.java:125)
            at org.junit.Assert.assertThrows(Assert.java:1001)
            ... 2 more