Fixing KeyErrors on messages without text

[D3N14L]

This may occur if messages consist only of media elements. A message
that consists just a gif for example

Fixes #1909

Status

READY

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• not tested

Checklist:

• I have performed a self-review of my own code
• [n/a] I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[codecov]

Codecov Report

Merging #1923 (74709e6) into master (2f2034a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1923   +/-   ##
=======================================
  Coverage   99.33%   99.33%           
=======================================
  Files          83       83           
  Lines        5114     5114           
=======================================
  Hits         5080     5080           
  Misses         34       34           

Impacted Files	Coverage Δ
opsdroid/connector/telegram/__init__.py	100.00% <ø> (ø)

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[FabioRosado]

Thank you for the fix!