[ENH] Documentation fixes and function changes for BaseCollectionEstimator

[MatthewMiddlehurst]

What does this implement/fix? Explain your changes.

• changes conversion function to return immediately if the input datatype is already in X_inner_type
• updates unequal length metadata to store None for unequal length data
• updates documentation for metadata and to match changes

[aeon-actions-bot]

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I have added the following labels to this PR based on the title: [ $\color{#FEF1BE}{\textsf{enhancement}}$ ].

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[TonyBagnall]

LGTM