formula_installer: handle nil req_dependency.
formula_installer: handle nil req_dependency.

Closes #3538.

aw1621107/brew,maxim-belkin/brew,ilovezfs/brew,alyssais/brew,hanxue/linuxbrew,hanxue/linuxbrew,claui/brew,nandub/brew,sjackman/homebrew,reelsense/brew,mgrimes/brew,pseudocody/brew,Linuxbrew/brew,JCount/brew,Homebrew/brew,konqui/brew,MikeMcQuaid/brew,ilovezfs/brew,mahori/brew,mahori/brew,pseudocody/brew,hanxue/linuxbrew,DomT4/brew,EricFromCanada/brew,claui/brew,JCount/brew,ilovezfs/brew,claui/brew,mahori/brew,aw1621107/brew,vitorgalvao/brew,DomT4/brew,konqui/brew,reitermarkus/brew,reitermarkus/brew,nandub/brew,Linuxbrew/brew,alyssais/brew,mgrimes/brew,vitorgalvao/brew,reelsense/brew,Homebrew/brew,Linuxbrew/brew,pseudocody/brew,MikeMcQuaid/brew,DomT4/brew,alyssais/brew,Homebrew/brew,nandub/brew,aw1621107/brew,sjackman/homebrew,EricFromCanada/brew,vitorgalvao/brew,vitorgalvao/brew,DomT4/brew,reitermarkus/brew,JCount/brew,EricFromCanada/brew,Homebrew/brew,MikeMcQuaid/brew,maxim-belkin/brew,Linuxbrew/brew,reitermarkus/brew,mgrimes/brew,maxim-belkin/brew,claui/brew,sjackman/homebrew,mahori/brew,konqui/brew,konqui/brew,EricFromCanada/brew,reelsense/brew,JCount/brew,sjackman/homebrew,MikeMcQuaid/brew,nandub/brew