Fix assignment to nil map in e2e scheduling predicates

Ready schedulable nodes are being inserted into an unitialized string
set, causing an assignment to entry in nil map in the underlying data
structure. This initializes the string set before attempting to insert
nodes.

Signed-off-by: hasheddan <[email protected]>

Author: hasheddan

test/e2e/scheduling/predicates.go

@@ -52,8 +52,8 @@ const (
 
 var localStorageVersion = utilversion.MustParseSemantic("v1.8.0-beta.0")
 
-// variable set in BeforeEach, never modified afterwards
-var workerNodes sets.String
+// variable populated in BeforeEach, never modified afterwards
+var workerNodes = sets.String{}
 
 type pausePodConfig struct {
 	Name                              string