Attempted fix for main-product/mask-product time extent mismatches.

[SpacemanPaul]

Problem: When data does exist for the main product, but does not exist for an associated tie-aware mask product, an unhandled error occurs.

This is an implementation of "solution 2" as discussed on Slack:

The question is what should OWS do in this case:

1. Act as if there is no data (return a blank tile); or
2. Render the data without masking.

This is consistent with the way the analogous issue is handled for ignore-time mask products.

[codecov]

Codecov Report

Merging #620 (7c5db58) into master (96eb442) will decrease coverage by 0.18%.
The diff coverage is 7.69%.

@@            Coverage Diff             @@
##           master     #620      +/-   ##
==========================================
- Coverage   89.05%   88.86%   -0.19%     
==========================================
  Files          43       43              
  Lines        5574     5587      +13     
==========================================
+ Hits         4964     4965       +1     
- Misses        610      622      +12     

Impacted Files	Coverage Δ
datacube_ows/data.py	79.35% <7.69%> (-1.72%)	⬇️