multiple reads of nasa-nex-gddp-cmip6 dataset from MultiZarrToZarr concatenated metadata returns all nans

Simply running twice cells 13 & 14 of the notebook that read and plot point single variable time-series for a point will reproduce this issue where the first run will have the valid values but second will be all nans. I encountered this when parallelizing reading of the files with dask that results in multiple reads and the unexpected result.

从 datasets/nasa-nex-gddp-cmip6/nasa-nex-gddp-cmip6-example.ipynb 中的单元格 13 和 14 开始，复现两次点时间序列读取。检查 MultiZarrToZarr 拼接元数据的行为，包括重复读取或并行读取。完成的标准是第二次读取返回与第一次相同的有效值，而不是全部为 NaNs。