fix: the children of <Collapse /> could be null

[ystarlongzi]

via Conditional makes rc-collapse crash on child.key

[coveralls]

Coverage increased (+0.09%) to 96.124% when pulling d6f846a on ystarlongzi:patch-1 into 312f1fb on react-component:master.

[shepherdwind]

rc-tools run lint
/home/travis/build/react-component/collapse/src/Collapse.jsx
   95:1  error  Trailing spaces not allowed  no-trailing-spaces
  120:1  error  Trailing spaces not allowed  no-trailing-spaces
✖ 2 problems (2 errors, 0 warnings)

eslint 修复下

[coveralls]

Coverage increased (+0.09%) to 96.124% when pulling 06a1d50 on ystarlongzi:patch-1 into 312f1fb on react-component:master.

[ystarlongzi]

@shepherdwind 修复了

[shepherdwind]

1.6.10