fix: table title can have null relationships

[alexbostock]

Fix for an error seen parsing a private document:

Block looks like this (some properties omitted)

{
    "BlockType": "TABLE_TITLE",
    "Page": 2,
    "PageClassification": null,
    "Query": null,
    "Relationships": null,
    "RowIndex": null,
    "RowSpan": null,
    "SelectionStatus": null,
    "Text": null,
    "TextType": null
}

Typically, a TABLE_TITLE block's relationships are words, but this title does not contain any text.

This could technically be a breaking change, if anybody relies on Relationships not being null. But I don't think this will be the case anywhere: several other block types have nullable relationships.