[fix] Fix crash when analyzer_command is null (#4823)

Old CodeChecker versions didn't store the analyzer command. In this case
`null` value was written to the analyzer_command column. Decompressing
it with zlib results an error.

Author: bruntib

web/server/codechecker_server/api/report_server.py

@@ -1885,7 +1885,7 @@ def getAnalysisInfo(self, analysis_info_filter, limit, offset):
 
                 for cmd in analysis_info_query:
                     command = zlib.decompress(cmd.analyzer_command) \
-                        .decode("utf-8")
+                        .decode("utf-8") if cmd.analyzer_command else ""
 
                     checkers_q = session \
                         .query(Checker.analyzer_name,