support lcov files without TN

Some lcov files don't have a `TN:` field.
The parser currently crashes in this case.
Just set `current_test_name` to empty at the beginning
to handle this case.

The documentation
https://manpages.ubuntu.com/manpages/noble/man1/geninfo.1.html
seems to say that the testname is indeed optional:

```
If available, a tracefile begins with the testname which is stored in the following format:

         TN:<test name>
```

fastcov.py

@@ -792,6 +792,7 @@ def parseInfo(path):
     }
 
     with open(path) as f:
+        current_test_name = ""
         for line in f:
             if line.startswith("TN:"):
                 current_test_name = line[3:].strip()

test/functional/expected_results/combine5.expected.info

@@ -0,0 +1,29 @@
+TN:
+SF:/mnt/workspace/test/functional/cmake_project/src/source1.cpp
+FN:3,_Z3foob
+FNDA:1,_Z3foob
+FNF:1
+FNH:1
+DA:3,1
+DA:5,1
+DA:7,1001
+DA:8,1000
+DA:10,1000
+DA:11,0
+DA:14,1
+LF:7
+LH:6
+end_of_record
+TN:
+SF:/mnt/workspace/test/functional/cmake_project/src/source2.cpp
+FN:3,_Z3barbii
+FNDA:10,_Z3barbii
+FNF:1
+FNH:1
+DA:3,10
+DA:5,10
+DA:6,10
+DA:8,0
+LF:4
+LH:3
+end_of_record

test/functional/expected_results/test1.no_tn.info

@@ -0,0 +1,27 @@
+SF:/mnt/workspace/test/functional/cmake_project/src/source1.cpp
+FN:3,_Z3foob
+FNDA:1,_Z3foob
+FNF:1
+FNH:1
+DA:3,1
+DA:5,1
+DA:7,1001
+DA:8,1000
+DA:10,1000
+DA:11,0
+DA:14,1
+LF:7
+LH:6
+end_of_record
+SF:/mnt/workspace/test/functional/cmake_project/src/source2.cpp
+FN:3,_Z3barbii
+FNDA:10,_Z3barbii
+FNF:1
+FNH:1
+DA:3,10
+DA:5,10
+DA:6,10
+DA:8,0
+LF:4
+LH:3
+end_of_record

test/functional/run_all.sh

@@ -129,6 +129,10 @@ cmp combine3.actual.info ${TEST_DIR}/expected_results/combine3.expected.info
 coverage run -a ${TEST_DIR}/fastcov.py -C ${TEST_DIR}/expected_results/combine4a.info ${TEST_DIR}/expected_results/combine4b.info ${TEST_DIR}/expected_results/combine4c.info --lcov -o combine4.actual.info
 cmp combine4.actual.info ${TEST_DIR}/expected_results/combine4.expected.info
 
+# Combine operation - Missing TN
+coverage run -a ${TEST_DIR}/fastcov.py -C ${TEST_DIR}/expected_results/test1.no_tn.info --lcov -o test1.no_tn.actual.info
+cmp test1.no_tn.actual.info ${TEST_DIR}/expected_results/combine5.expected.info
+
 # Run ctest_2
 ${TEST_DIR}/fastcov.py --gcov gcov-9 --zerocounters  # Clear previous test coverage
 ctest -R ctest_2