PP: Fix issue #407; handle empty identifier.

The sequence

  #define m()
  int m"

creates a token of no length (a string of 0 size).  Protect
against a string of 0 size as well as the existing protect
against a null string.

Test/baseResults/cppBad.vert.out

@@ -1,8 +1,10 @@
 cppBad.vert
 ERROR: 0:2: 'preprocessor evaluation' : bad expression 
 ERROR: 0:2: '#if' : unexpected tokens following directive 
-ERROR: 0:3: '' : missing #endif 
-ERROR: 3 compilation errors.  No code generated.
+ERROR: 0:5: 'string' : End of line in string 
+ERROR: 0:5: 'macro expansion' : expected '(' following n
+ERROR: 0:5: '' :  syntax error
+ERROR: 5 compilation errors.  No code generated.
 
 
 Shader version: 100