Fix aggregation for empty documents

[percevalw]

Description

This PR fixes aggregation issues for empty documents.

Checklist

• If this PR is a bug fix, the bug is documented in the test suite.
• Changes were documented in the changelog (pending section).
• If necessary, changes were made to the documentation.

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (d8ed766) compared to base (20b466d).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##            master        #8   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           33        33           
  Lines          584       588    +4     
=========================================
+ Hits           584       588    +4     

Impacted Files	Coverage Δ
edspdf/aggregators/simple.py	100.00% <100.00%> (ø)
edspdf/aggregators/styled.py	100.00% <100.00%> (ø)

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