Fix printing of empty types

[vhenzl]

See graphql/graphql-js#1707

Resolves one of the problems responsible for many test failures in #928 and #929.

[codecov]

Codecov Report

Merging #940 (cec4c36) into master (6e80894) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head cec4c36 differs from pull request most recent head 796a788. Consider uploading reports for the commit 796a788 to get more accurate results

@@            Coverage Diff            @@
##             master     #940   +/-   ##
=========================================
  Coverage     94.23%   94.23%           
  Complexity       50       50           
=========================================
  Files           117      117           
  Lines          9674     9683    +9     
=========================================
+ Hits           9116     9125    +9     
  Misses          558      558           

Impacted Files	Coverage Δ
src/Utils/SchemaPrinter.php	99.04% <100.00%> (+0.10%)	⬆️
src/Language/BlockString.php	100.00% <0.00%> (ø)
src/Language/Printer.php	99.28% <0.00%> (+0.02%)	⬆️

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[spawnia]

Superseded by #941?

[vhenzl]

Superseded by #941?

No, they are independent changes.