cmTarget: Don't crash on empty IMPORTED_CONFIGURATIONS

The logic to select an imported configuration handled the case when
`IMPORTED_CONFIGURATIONS` was missing, but assumed that if it was
present, it was non-empty, which could cause CMake to crash if that
happens. (To be fair, I believe this would never happen for imports
generated by CMake.) Adjust the logic slightly to follow the same code
path when `IMPORTED_CONFIGURATIONS` is empty as is taken if it's
missing.

Fixes: #28065

Source/cmTarget.cxx

@@ -3580,7 +3580,8 @@ bool cmTarget::GetMappedConfigNew(std::string desiredConfig, cmValue& loc,
   }
 
   // Get imported configurations, if specified.
-  if (cmValue iconfigs = this->GetProperty("IMPORTED_CONFIGURATIONS")) {
+  cmValue const iconfigs = this->GetProperty("IMPORTED_CONFIGURATIONS");
+  if (!iconfigs.IsEmpty()) {
     cmList const availableConfigs{ cmSystemTools::UpperCase(*iconfigs) };
 
     if (!mappedConfigs.empty()) {

Tests/RunCMake/GeneratorExpression/Empty-IMPORTED_CONFIGURATIONS-result.txt

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

Tests/RunCMake/GeneratorExpression/Empty-IMPORTED_CONFIGURATIONS-stderr.txt

@@ -0,0 +1,6 @@
+(CMake Error in CMakeLists.txt:
+  IMPORTED_LOCATION not set for imported target "imported"( configuration
+  "[A-Za-z]+")?\.
+
+)+
+CMake Generate step failed\.  Build files cannot be regenerated correctly\.

Tests/RunCMake/GeneratorExpression/Empty-IMPORTED_CONFIGURATIONS.cmake

@@ -0,0 +1,7 @@
+cmake_minimum_required(VERSION 4.4)
+project(empty_imported_configuration NONE)
+
+add_library(imported UNKNOWN IMPORTED)
+set_property(TARGET imported PROPERTY IMPORTED_CONFIGURATIONS "")
+
+file(GENERATE OUTPUT out.txt CONTENT "$<TARGET_FILE:imported>")

Tests/RunCMake/GeneratorExpression/RunCMakeTest.cmake

@@ -160,6 +160,8 @@ run_cmake_build(CMP0200-NEW)
 
 run_cmake_build(CMP0199-NEW+CMP0200-NEW)
 
+run_cmake(Empty-IMPORTED_CONFIGURATIONS)
+
 set(RunCMake_TEST_OPTIONS -DCMAKE_POLICY_DEFAULT_CMP0085:STRING=OLD)
 run_cmake(CMP0085-OLD)
 unset(RunCMake_TEST_OPTIONS)