build: Fix error during install in offline builds

cmake/build_helpers.cmake

@@ -641,10 +641,12 @@ function(downloadImHexPatternsFiles dest)
             endforeach()
         ]])
     else()
+        if (NOT (imhex_patterns_SOURCE_DIR STREQUAL ""))
             set(PATTERNS_FOLDERS_TO_INSTALL constants encodings includes patterns magic nodes)
             foreach (FOLDER ${PATTERNS_FOLDERS_TO_INSTALL})
                 install(DIRECTORY "${imhex_patterns_SOURCE_DIR}/${FOLDER}" DESTINATION "${dest}" PATTERN "**/_schema.json" EXCLUDE)
             endforeach ()
+        endif()
     endif ()
 
 endfunction()