adjacent returns false if keys are undefined #55
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this is a weird bug that i noticed while writing an eslint rule with the following selector
for certain types of file using babel-eslint, this would throw
Cannot read property 'length' of undefinedDetails
``` Cannot read property 'length' of undefined TypeError: Cannot read property 'length' of undefined at adjacent (/Users/marcos/SpacedOut/sense/ui_server/node_modules/esquery/esquery.js:214:33) at Function.matches (/Users/marcos/SpacedOut/sense/ui_server/node_modules/esquery/esquery.js:135:25) at NodeEventGenerator.applySelector (/Users/marcos/SpacedOut/sense/ui_server/node_modules/eslint/lib/util/node-event-generator.js:264:21) at NodeEventGenerator.applySelectors (/Users/marcos/SpacedOut/sense/ui_server/node_modules/eslint/lib/util/node-event-generator.js:292:22) at NodeEventGenerator.enterNode (/Users/marcos/SpacedOut/sense/ui_server/node_modules/eslint/lib/util/node-event-generator.js:308:14) at CodePathAnalyzer.enterNode (/Users/marcos/SpacedOut/sense/ui_server/node_modules/eslint/lib/code-path-analysis/code-path-analyzer.js:602:23) at CommentEventGenerator.enterNode (/Users/marcos/SpacedOut/sense/ui_server/node_modules/eslint/lib/util/comment-event-generator.js:98:23) at Traverser.enter (/Users/marcos/SpacedOut/sense/ui_server/node_modules/eslint/lib/eslint.js:929:36) at Traverser.__execute (/Users/marcos/SpacedOut/sense/ui_server/node_modules/estraverse/estraverse.js:397:31) at Traverser.traverse (/Users/marcos/SpacedOut/sense/ui_server/node_modules/estraverse/estraverse.js:501:28) ```adding this fixes my specific bug but i have to suspect that it's possible that other selectors are vulnerable to this problem. i can amend this pr to catch those cases as well if you think it's a worthwhile change.