Fix use_existing tensors bugs

[farshidz]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Bug fix

• What is the current behavior? (You can also link to an open issue here)
  Add docs use_existing_tensords:

  • Does not pick up existing vectors and revectorises regardless
  • Fails when reindexing a doc with no change and a multimodal field
  • Fails when existing doc does not have a multimodal field, but the new doc being indexed does (unstructured only)
  • Reindexing an unstructured document with a multimodal field adds the wrong chunk values by adding a list to a list

Also: get_documents_by_ids fails when a document has no vectors

• What is the new behavior (if this is a feature change)?
  Issues above fixed

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

• Have unit tests been run against this PR? (Has there also been any additional testing?)
  Running

• Related Python client changes (link commit/PR here)
  None

• Related documentation changes (link commit/PR here)
  None

• Other information:

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added / updated (for bug fixes / features)