Do not assume the cell will be present
Do not assume the cell will be present

Summary: When parsing cross-cell targets really early, the cell might not be registered yet. If that is the case, we end up with a `null` cell and throw an error when we should not

Test Plan: `buck targets xplat//third-party/build-tools:` when those BUCK files look through symlinks

Reviewed By: ttsugriy

fbshipit-source-id: b109bf1

brettwooldridge/buck,clonetwin26/buck,JoelMarcey/buck,shs96c/buck,LegNeato/buck,romanoid/buck,clonetwin26/buck,brettwooldridge/buck,JoelMarcey/buck,LegNeato/buck,clonetwin26/buck,shs96c/buck,clonetwin26/buck,rmaz/buck,shs96c/buck,SeleniumHQ/buck,Addepar/buck,JoelMarcey/buck,LegNeato/buck,facebook/buck,SeleniumHQ/buck,brettwooldridge/buck,facebook/buck,shs96c/buck,SeleniumHQ/buck,clonetwin26/buck,nguyentruongtho/buck,clonetwin26/buck,brettwooldridge/buck,nguyentruongtho/buck,shs96c/buck,shs96c/buck,JoelMarcey/buck,brettwooldridge/buck,rmaz/buck,Addepar/buck,romanoid/buck,facebook/buck,nguyentruongtho/buck,romanoid/buck,LegNeato/buck,zpao/buck,romanoid/buck,JoelMarcey/buck,JoelMarcey/buck,romanoid/buck,brettwooldridge/buck,rmaz/buck,LegNeato/buck,brettwooldridge/buck,zpao/buck,shs96c/buck,clonetwin26/buck,Addepar/buck,rmaz/buck,JoelMarcey/buck,clonetwin26/buck,rmaz/buck,romanoid/buck,kageiit/buck,brettwooldridge/buck,clonetwin26/buck,rmaz/buck,clonetwin26/buck,zpao/buck,JoelMarcey/buck,JoelMarcey/buck,shs96c/buck,zpao/buck,brettwooldridge/buck,LegNeato/buck,Addepar/buck,rmaz/buck,Addepar/buck,rmaz/buck,romanoid/buck,zpao/buck,LegNeato/buck,clonetwin26/buck,rmaz/buck,romanoid/buck,JoelMarcey/buck,SeleniumHQ/buck,Addepar/buck,facebook/buck,LegNeato/buck,brettwooldridge/buck,romanoid/buck,SeleniumHQ/buck,brettwooldridge/buck,romanoid/buck,SeleniumHQ/buck,brettwooldridge/buck,kageiit/buck,Addepar/buck,facebook/buck,SeleniumHQ/buck,Addepar/buck,brettwooldridge/buck,kageiit/buck,nguyentruongtho/buck,shs96c/buck,LegNeato/buck,LegNeato/buck,SeleniumHQ/buck,rmaz/buck,shs96c/buck,nguyentruongtho/buck,facebook/buck,zpao/buck,kageiit/buck,nguyentruongtho/buck,shs96c/buck,facebook/buck,SeleniumHQ/buck,romanoid/buck,Addepar/buck,Addepar/buck,nguyentruongtho/buck,LegNeato/buck,clonetwin26/buck,JoelMarcey/buck,SeleniumHQ/buck,Addepar/buck,rmaz/buck,Addepar/buck,rmaz/buck,romanoid/buck,LegNeato/buck,LegNeato/buck,SeleniumHQ/buck,kageiit/buck,zpao/buck,kageiit/buck,rmaz/buck,JoelMarcey/buck,SeleniumHQ/buck,kageiit/buck,romanoid/buck,shs96c/buck,shs96c/buck,JoelMarcey/buck,Addepar/buck,clonetwin26/buck,SeleniumHQ/buck