Use emptiness-checks rather than DEFINED checks

For the MBEDTLS_CONFIG_FILE and MBEDTLS_USER_CONFIG_FILE variables,
check that they are non-empty and defined. This means they can be
unconditionally created in the cache, simplifying the CMakeLists.txt

Signed-off-by: David Horstmann <david.horstmann@arm.com>
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 61532ff..a159251 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -230,11 +230,11 @@
     target_compile_definitions(${target}
         PRIVATE ${thirdparty_def})
     # Pass-through MBEDTLS_CONFIG_FILE and MBEDTLS_USER_CONFIG_FILE
-    if(DEFINED MBEDTLS_CONFIG_FILE)
+    if(MBEDTLS_CONFIG_FILE)
         target_compile_definitions(${target}
             PUBLIC MBEDTLS_CONFIG_FILE="${MBEDTLS_CONFIG_FILE}")
     endif()
-    if(DEFINED MBEDTLS_USER_CONFIG_FILE)
+    if(MBEDTLS_USER_CONFIG_FILE)
         target_compile_definitions(${target}
             PUBLIC MBEDTLS_USER_CONFIG_FILE="${MBEDTLS_USER_CONFIG_FILE}")
     endif()