CachingUtilities.py: Replace "==" with "is"
CachingUtilities.py: Replace "==" with "is"

Replaces "==" with "is" to check if "file_path" is
"None", to comply with PEP8.

Closes https://github.com/coala/coala/issues/2920

yashLadha/coala,rimacone/testing2,ayushin78/coala,shreyans800755/coala,shreyans800755/coala,kartikeys98/coala,karansingh1559/coala,incorrectusername/coala,aptrishu/coala,arush0311/coala,jayvdb/coala,sagark123/coala,sils1297/coala,yashtrivedi96/coala,yashLadha/coala,arjunsinghy96/coala,SanketDG/coala,refeed/coala,RJ722/coala,MariosPanag/coala,Asalle/coala,Nosferatul/coala,NiklasMM/coala,kartikeys98/coala,nemaniarjun/coala,coala-analyzer/coala,Asnelchristian/coala,shreyans800755/coala,karansingh1559/coala,damngamerz/coala,NiklasMM/coala,Shade5/coala,nemaniarjun/coala,arush0311/coala,impmihai/coala,kartikeys98/coala,rimacone/testing2,damngamerz/coala,coala-analyzer/coala,Balaji2198/coala,ayushin78/coala,Nosferatul/coala,Shade5/coala,MariosPanag/coala,yashtrivedi96/coala,yashLadha/coala,sils1297/coala,yashtrivedi96/coala,nemaniarjun/coala,sagark123/coala,jayvdb/coala,netman92/coala,coala-analyzer/coala,CruiseDevice/coala,refeed/coala,aptrishu/coala,impmihai/coala,netman92/coala,CruiseDevice/coala,coala/coala,Shade5/coala,sagark123/coala,aptrishu/coala,CruiseDevice/coala,Asnelchristian/coala,rimacone/testing2,karansingh1559/coala,coala/coala,arush0311/coala,MariosPanag/coala,meetmangukiya/coala,Asnelchristian/coala,meetmangukiya/coala,Balaji2198/coala,impmihai/coala,jayvdb/coala,JohnS-01/coala,RJ722/coala,arjunsinghy96/coala,Balaji2198/coala,incorrectusername/coala,JohnS-01/coala,coala/coala,SanketDG/coala,NiklasMM/coala,sils1297/coala,ayushin78/coala,SanketDG/coala,netman92/coala,damngamerz/coala,Asalle/coala,incorrectusername/coala,refeed/coala,JohnS-01/coala,meetmangukiya/coala,Nosferatul/coala,RJ722/coala,Asalle/coala,arjunsinghy96/coala