c# core: added null check for keys
c# core: added null check for keys

grokify/c-sharp,grokify/c-sharp,grokify/c-sharp,grokify/c-sharp,marceloinacio/c-sharp,marceloinacio/c-sharp,grokify/c-sharp,marceloinacio/c-sharp,marceloinacio/c-sharp,marceloinacio/c-sharp