GenEx: Parse comma after colon tokens specially

Otherwise the comma is treated as plain text by ParseContent.

$<STREQUAL:,> should be valid and true.
$<STREQUAL:,something> should be valid and false.

$<STREQUAL:,,> should be non-valid as it is 3 parameters.
$<STREQUAL:something,,> should be non-valid as it is 3 parameters.

Additionally, this allows reporting the correct error for other
expressions. For example $<TARGET_PROPERTY:,> should be invalid
because it has an empty target and empty property. It shouldn't
attempt to read the property ',' on the 'implicit this' target.

Tests/GeneratorExpression/CMakeLists.txt

@@ -37,6 +37,8 @@ add_custom_target(check ALL
     -Dtest_strequal_angle_r=$<STREQUAL:$<ANGLE-R>,$<ANGLE-R>>
     -Dtest_strequal_comma=$<STREQUAL:$<COMMA>,$<COMMA>>
     -Dtest_strequal_angle_r_comma=$<STREQUAL:$<ANGLE-R>,$<COMMA>>
+    -Dtest_strequal_both_empty=$<STREQUAL:,>
+    -Dtest_strequal_one_empty=$<STREQUAL:something,>
     -Dtest_angle_r=$<ANGLE-R>
     -Dtest_comma=$<COMMA>
     -P ${CMAKE_CURRENT_SOURCE_DIR}/check.cmake

Tests/GeneratorExpression/check.cmake

@@ -38,5 +38,7 @@ check(test_strequal_no_yes "0")
 check(test_strequal_angle_r "1")
 check(test_strequal_comma "1")
 check(test_strequal_angle_r_comma "0")
+check(test_strequal_both_empty "1")
+check(test_strequal_one_empty "0")
 check(test_angle_r ">")
 check(test_comma ",")