Make database file optional for EpicsAdapter

[codecov[bot]]

Codecov Report

Merging #134 (6dc6cd4) into master (6483aac) will increase coverage by 0.74%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
+ Coverage   92.48%   93.23%   +0.74%     
==========================================
  Files          45       45              
  Lines        1225     1226       +1     
==========================================
+ Hits         1133     1143      +10     
+ Misses         92       83       -9     

Impacted Files	Coverage Δ
src/tickit/adapters/epicsadapter/adapter.py	100.00% <100.00%> (+12.24%)	:arrow_up:

... and 1 file with indirect coverage changes

[joeshannon]

Yeah that sounds like a good idea. Will take a look at that tomorrow.