cmGeneratorExpressionParser: Remove dead "if empty" check

Add a test case to verify preservation of the existing behavior, which
is an evaluation error.

Source/cmGeneratorExpressionParser.cxx

@@ -82,9 +82,6 @@ void cmGeneratorExpressionParser::ParseGeneratorExpression(
       break;
     }
   }
-  if (identifier.empty()) {
-    // ERROR
-  }
 
   if (this->it != this->Tokens.end() &&
       this->it->TokenType == cmGeneratorExpressionToken::EndExpression) {

Tests/RunCMake/GeneratorExpression/BadEmpty-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/GeneratorExpression/BadEmpty-stderr.txt

@@ -0,0 +1,17 @@
+CMake Error at BadEmpty\.cmake:1 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<>
+
+  Expression did not evaluate to a known generator expression
+Call Stack \(most recent call first\):
+  CMakeLists\.txt:[0-9]+ \(include\)
++
+CMake Error at BadEmpty\.cmake:1 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<:foo>
+
+  Expression did not evaluate to a known generator expression
+Call Stack \(most recent call first\):
+  CMakeLists\.txt:[0-9]+ \(include\)

Tests/RunCMake/GeneratorExpression/BadEmpty.cmake

@@ -0,0 +1,4 @@
+add_custom_target(check ALL COMMAND check
+  $<>
+  $<:foo>
+  VERBATIM)

Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake

@@ -20,6 +20,7 @@ run_cmake(STRLESS_EQUAL)
 run_cmake(STRGREATER)
 run_cmake(STRGREATER_EQUAL)
 run_cmake(BadZero)
+run_cmake(BadEmpty)
 run_cmake(BadTargetName)
 run_cmake(BadTargetTypeInterface)
 run_cmake(BadTargetTypeObject)