Only include link to source if one has been provided
Only include link to source if one has been provided

Not all licenses have provided a `source` property, so this check just
prevent an empty `href` in such cases.

christianbundy/choosealicense.com,github/choosealicense.com,JTechMe/choosealicense.com,JTechMe/choosealicense.com,drewwyatt/choosealicense.com,mickaelandrieu/choisirunelicence,julianromera/choosealicense.com,cirosantilli/choosealicense.com,drewwyatt/choosealicense.com,drewwyatt/choosealicense.com,gorcz/choosealicense.com,julianromera/choosealicense.com,matt40k/choosealicense.com,github/choosealicense.com,shalomb/choosealicense.com,mickaelandrieu/choisirunelicence,sieversMartin/choosealicense.com,matt40k/choosealicense.com,gorcz/choosealicense.com,sieversMartin/choosealicense.com,gorcz/choosealicense.com,n7s/choosealicense.com,derekjones/choosealicense.com,derekjones/choosealicense.com,cirosantilli/choosealicense.com,liks79/choosealicense.com,christianbundy/choosealicense.com,derekjones/choosealicense.com,matt40k/choosealicense.com,mickaelandrieu/choisirunelicence,github/choosealicense.com,liks79/choosealicense.com,sieversMartin/choosealicense.com,waldyrious/choosealicense.com,eogas/choosealicense.com,shalomb/choosealicense.com,waldyrious/choosealicense.com,JTechMe/choosealicense.com,cirosantilli/choosealicense.com,christianbundy/choosealicense.com,n7s/choosealicense.com,waldyrious/choosealicense.com,shalomb/choosealicense.com,n7s/choosealicense.com,eogas/choosealicense.com,julianromera/choosealicense.com,eogas/choosealicense.com