fix(type): allow nullable value for OptionDataValue

[avin-kavish]

FYI, as per the doc null and undefined values are accepted in series.data. But the types do not allow it.

Empty value:
'-' or null or undefined or NaN can be used to describe that a data item does not exist (ps：not exist does not means its value is 0).
For example, line chart can break when encounter an empty value, and scatter chart do not display graphic elements for empty values.

This is mostly a FYI PR, haven't done any dues on how this will affect the rest of the types. Tests etc.

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The changes brought by this PR can be previewed at: https://echarts.apache.org/examples/editor?version=PR-19111@33b437d

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