predict_curve: `labels` should not be optional

[LukeWeidenwalker]

Process ID: predict_curve

Describe the issue: The docs for the labels parameter of predict_curve say this:

"The labels to predict values for. If no labels are given, predicts values only for no-data (null) values in the data cube."

However, in the 2.0.0 version of predict_curve, there's no longer the option to pass in a datacube, so labels shouldn't be allowed to be null.

Proposed solution: Make labels a required parameter for this process and remove the section about filling in nodata-values from the documentation.

[m-mohr]

Happy to review a PR.