Added null check for the query.getKey();

[Sarkie]

Fixes issue #21

[Sarkie]

Or could use an argument exception

for (PostmanQuery query : postmanItem.getRequest().getUrl().getQueries()) {
            argument = new HTTPArgument();
			if (query.getKey() == null){
				throw new IllegalArgumentException(postmanItem.getRequest().getUrl().getRaw() + " has a null query");
			}
            argument.setName(query.getKey());
            argument.setValue(query.getValue());
            argument.setDescription(query.getKey());
            argument.setEnabled(true);
            argument.setMetaData("=");
            argument.setAlwaysEncoded(false);
            argument.setUseEquals(true);
            arguments.addArgument(argument);
        }
        httpSamplerProxy.setArguments(arguments);

        return httpSamplerProxy;