memory: Add constructor for empty device_unique_object

[stotko]

In #391, a new wrapper class for automatically managing device objects has been introduced. However, empty managed object can not be constructed which prohibits use cases where, e.g., such a managed object is moved. Add a respective constructor that accepts a null_object as well as an overload of operator bool() to enable testing for empty objects.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (50530de) 97.40% compared to head (ae6ead0) 97.40%.

❗ Current head ae6ead0 differs from pull request most recent head 99589ee. Consider uploading reports for the commit 99589ee to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #402   +/-   ##
=======================================
  Coverage   97.40%   97.40%           
=======================================
  Files          31       31           
  Lines        2581     2586    +5     
=======================================
+ Hits         2514     2519    +5     
  Misses         67       67           

Files Changed	Coverage Δ
src/stdgpu/impl/memory_detail.h	99.11% <100.00%> (+0.02%)	⬆️

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