[Android] Fix C++ Crash in WeexCore::NotifyLayout

[YorkShen]

The return value of jString2StrFast(env, instanceId) could be std::string(nullptr)`` if instanceId is nullptr`, in which case the return value would cause crash.

    #00 pc 0x4aaec libweexcore.so (_ZNSt6__ndk16__treeINS_12__value_typeINS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEEPN8WeexCore14RenderPageBaseEEENS_19__map_value_compareIS7_SB_NS_4lessIS7_EELb1EEENS5_ISB_EEE13__lower_boundIS7_EENS_15__tree_iteratorISB_PNS_11__tree_nodeISB_PvEEiEERKT_SN_PNS_15__tree_end_nodeIPNS_16__tree_node_baseISL_EEEE+23)
    #01 pc 0x4aa6b libweexcore.so (_ZNSt6__ndk16__treeINS_12__value_typeINS_12basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEEEPN8WeexCore14RenderPageBaseEEENS_19__map_value_compareIS7_SB_NS_4lessIS7_EELb1EEENS5_ISB_EEE4findIS7_EENS_15__tree_iteratorISB_PNS_11__tree_nodeISB_PvEEiEERKT_+18)
    #02 pc 0x4907b libweexcore.so (_ZN8WeexCore13RenderManager7GetPageERKNSt6__ndk112basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEE+6)
    #03 pc 0x5d2d5 libweexcore.so (_ZN8WeexCore18CoreSideInPlatform12NotifyLayoutERKNSt6__ndk112basic_stringIcNS1_11char_traitsIcEENS1_9allocatorIcEEEE+12)

Brief Description of the PR

Checklist

• Demo:
• Documentation:

[weex-bot]

	Warnings
⚠️	No Changelog changes! - Can you add a Changelog? To do so,append your changes to the changelog.md
⚠️	If your PR is about fixing a bug excluding crash the code,you should add the demo link in the PR description. Demo link: http://dotwe.org/vue
⚠️	If you update the code, maybe you should update the documentation and add the documentation link in the PR description. here is the guide about how to contribute documentation: https://github.com/apache/incubator-weex/blob/master/CONTRIBUTING.md#contribute-code-or-document

	Messages
📖	danger test finished.

Generated by 🚫 dangerJS against 2da0847

[jianhan-he]

LGTM