An empty stem crashes antora

the following code crashes antora with a less than helpful message about where to look

[stem]
++++

++++

the message is:

error: undefined method `start_with?' for nil
Add the --stacktrace option to see the cause.

with the stacktrace:

start_with?: undefined method `start_with?' for nil
    at $NilClass.$$method_missing (/Users/prudhomm/.nvm/versions/node/v12.18.3/lib/node_modules/@antora/site-generator-default/node_modules/opal-runtime/src/opal.js:3910:56)
    at $NilClass.method_missing_stub (/Users/prudhomm/.nvm/versions/node/v12.18.3/lib/node_modules/@antora/site-generator-default/node_modules/opal-runtime/src/opal.js:1299:35)
    at Object.$$stem (/Users/prudhomm/.nvm/versions/node/v12.18.3/lib/node_modules/@antora/site-generator-default/node_modules/asciidoctor.js/dist/node/asciidoctor.js:16238:59)
    at Object.$$__send__ (/Users/prudhomm/.nvm/versions/node/v12.18.3/lib/node_modules/@antora/site-generator-default/node_modules/opal-runtime/src/opal.js:3705:21)
    at Object.Opal.send (/Users/prudhomm/.nvm/versions/node/v12.18.3/lib/node_modules/@antora/site-generator-default/node_modules/opal-runtime/src/opal.js:1660:19)
    at Object.send (/Users/prudhomm/.nvm/versions/node/v12.18.3/lib/node_modules/@antora/site-generator-default/node_modules/opal-runtime/src/opal.js:1850:19)
    at Object.$$convert (/Users/prudhomm/.nvm/versions/node/v12.18.3/lib/node_modules/@antora/site-generator-default/node_modules/asciidoctor.js/dist/node/asciidoctor.js:6215:23)
    at $Block.$$convert (/Users/prudhomm/.nvm/versions/node/v12.18.3/lib/node_modules/@antora/site-generator-default/node_modules/asciidoctor.js/dist/node/asciidoctor.js:5192:34)
    at TMP_8 (/Users/prudhomm/.nvm/versions/node/v12.18.3/lib/node_modules/@antora/site-generator-default/node_modules/asciidoctor.js/dist/node/asciidoctor.js:5206:20)
    at Object.Opal.yield1 (/Users/prudhomm/.nvm/versions/node/v12.18.3/lib/node_modules/@antora/site-generator-default/node_modules/opal-runtime/src/opal.js:1441:14)

there is this $$stem that may indicate that the issue is with a stem.