Fix: no-boolean-default with 'default-false' on boolean props with unset default

[ghost]

Currently the rule no-boolean-default breaks when used with option 'default-false' on a boolean prop that does not have default property, with exception:

TypeError: Cannot read property 'value' of undefined
      at booleanProps.forEach.propDef (lib/rules/no-boolean-default.js:7:2342)
      at Array.forEach (<anonymous>)
      at utils.executeOnVueComponent.obj (lib/rules/no-boolean-default.js:7:1808)
      at ExportDefaultDeclaration:exit (lib/utils/index.js:174:251)
      at listeners.(anonymous function).forEach.listener (node_modules/eslint/lib/util/safe-emitter.js:45:58)
      at Array.forEach (<anonymous>)
      at Object.emit (node_modules/eslint/lib/util/safe-emitter.js:45:38)
      at NodeEventGenerator.applySelector (node_modules/eslint/lib/util/node-event-generator.js:251:26)
      at NodeEventGenerator.applySelectors (node_modules/eslint/lib/util/node-event-generator.js:280:22)
      at NodeEventGenerator.leaveNode (node_modules/eslint/lib/util/node-event-generator.js:303:14)
      at CodePathAnalyzer.leaveNode (node_modules/eslint/lib/code-path-analysis/code-path-analyzer.js:654:23)
      at nodeQueue.forEach.traversalInfo (node_modules/eslint/lib/linter.js:752:28)
      at Array.forEach (<anonymous>)
      at runRules (node_modules/eslint/lib/linter.js:746:15)
      at Linter._verifyWithoutProcessors (node_modules/eslint/lib/linter.js:891:31)
      at preprocess.map.textBlock (node_modules/eslint/lib/linter.js:940:35)
      at Array.map (<anonymous>)
      at Linter.verify (node_modules/eslint/lib/linter.js:939:42)
      at runRuleForItem (node_modules/eslint/lib/testers/rule-tester.js:385:34)
      at testValidTemplate (node_modules/eslint/lib/testers/rule-tester.js:412:28)
      at Context.RuleTester.it (node_modules/eslint/lib/testers/rule-tester.js:584:25)

This PR adds tests that expose the issue (simply the same cases but applied without options and with each option where applicable) and a check for a falsy value returned from getDefaultNode(propDef).

Also, changed the error message for 'default-false' option a bit, as "Boolean prop should be defaulted to false." may mean that it "should be defaulted", while the logic is "can be defaulted or not, but if it is, it must only be false".

[mysticatea]

LGTM, thank you so much!