s3: Empty routing rules refused by AWS

[chlunde]

Description of your changes

When no routing rules are defined for a website configuration, we must
avoid sending an empty XML attribute as AWS refuses this as part of the
schema validation:

<WebsiteConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/"><ErrorDocument><Key>techdocs-docs/index.html</Key></ErrorDocument><IndexDocument><Suffix>index.html</Suffix></IndexDocument><RoutingRules></RoutingRules></WebsiteConfiguration>

cannot create or update: api error MalformedXML: The XML you provided was not well-formed or did not validate against our published schema

for the input:

websiteConfiguration:
  errorDocument:
    key: 'techdocs-docs/index.html'
  indexDocument:
    suffix: 'index.html'

Fixes bug reported on slack. #1155

I have:

• Read and followed Crossplane's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

Created bucket, verified in the console that the config was created as requested.

apiVersion: s3.aws.crossplane.io/v1beta1
kind: Bucket
metadata:
  name: test-bucket
  annotations:
    # This will be the actual bucket name. It must be globally unique, so you
    # probably want to change it before trying to apply this example.
    crossplane.io/external-name: crossplane-example-bucket123
spec:
  forProvider:
    acl: private
    locationConstraint: us-east-1
    publicAccessBlockConfiguration:
      blockPublicPolicy: true
    websiteConfiguration:
      errorDocument: 
        key: 'techdocs-docs/index.html'
      indexDocument: 
        suffix: 'index.html'

[haarchri]

LGTM - we have the same running internally ;) missed to upstream