Fix issue where setSlashApiKey would break if no headers was set.

[Sydney-o9]

Hi Team,

See https://discuss.dgraph.io/t/dgraph-js-http-setslashapikey-breaks/11250

This change is 

[CLAassistant]

All committers have signed the CLA.

[MichelDiz]

Hey @Sydney-o9 , there are some warnings on your PR

ERROR: 375:9   no-unused-expression        unused expression, expected an assignment or function call
ERROR: 375:47  strict-boolean-expressions  This type is not allowed in the operand for the '&&' operator because it is always truthy. It may be null/undefined, but neither 'allow-null-union' nor 'allow-undefined-union' is set. Only booleans are allowed.
ERROR: 380:9   no-unused-expression        unused expression, expected an assignment or function call
ERROR: 380:47  strict-boolean-expressions  This type is not allowed in the operand for the '&&' operator because it is always truthy. It may be null/undefined, but neither 'allow-null-union' nor 'allow-undefined-union' is set. Only booleans are allowed.

[gja]

This LGTM. Let's remove if boolean short circuits on 375 and 380 and just make them if conditions. @vardhanapoorv could you take this forward?